Fix "unexpected heading" Javadoc errors on Java 13+.

Author: PascalSchumacher

src/main/java/org/apache/commons/io/DirectoryWalker.java

@@ -46,7 +46,7 @@
  *          behaviour.</li>
  *   </ul>
  *
- * <h3 id="example">1. Example Implementation</h3>
+ * <h2 id="example">1. Example Implementation</h2>
  *
  * There are many possible extensions, for example, to delete all
  * files and '.svn' directories, and return a list of deleted files:
@@ -82,7 +82,7 @@
  *  }
  * </pre>
  *
- * <h3 id="filter">2. Filter Example</h3>
+ * <h2 id="filter">2. Filter Example</h2>
  *
  * <p>
  * Choosing which directories and files to process can be a key aspect
@@ -157,7 +157,7 @@
  * option for filtering.
  * </p>
  *
- * <h3 id="cancel">3. Cancellation</h3>
+ * <h2 id="cancel">3. Cancellation</h2>
  *
  * <p>
  * The DirectoryWalker contains some of the logic required for cancel processing.
@@ -195,7 +195,7 @@
  * scenarios.
  * </p>
  *
- * <h4 id="external">3.1 External / Multi-threaded</h4>
+ * <h3 id="external">3.1 External / Multi-threaded</h3>
  *
  * <p>
  * This example provides a public <code>cancel()</code> method that can be
@@ -228,7 +228,7 @@
  *  }
  * </pre>
  *
- * <h4 id="internal">3.2 Internal</h4>
+ * <h3 id="internal">3.2 Internal</h3>
  *
  * <p>
  * This shows an example of how internal cancellation processing could be implemented.

src/main/java/org/apache/commons/io/FileUtils.java

@@ -1309,13 +1309,13 @@ public static void copyDirectory(final File srcDir, final File destDir,
      * it is not guaranteed that those operations will succeed.
      * If the modification operation fails, no indication is provided.
      * </p>
-     * <h3>Example: Copy directories only</h3>
+     * <h4>Example: Copy directories only</h4>
      * <pre>
      *  // only copy the directory structure
      *  FileUtils.copyDirectory(srcDir, destDir, DirectoryFileFilter.DIRECTORY);
      *  </pre>
      *
-     * <h3>Example: Copy directories and txt files</h3>
+     * <h4>Example: Copy directories and txt files</h4>
      * <pre>
      *  // Create a filter for ".txt" files
      *  IOFileFilter txtSuffixFilter = FileFilterUtils.suffixFileFilter(".txt");
@@ -1361,13 +1361,13 @@ public static void copyDirectory(final File srcDir, final File destDir,
      * not guaranteed that those operations will succeed.
      * If the modification operation fails, no indication is provided.
      * </p>
-     * <h3>Example: Copy directories only</h3>
+     * <h4>Example: Copy directories only</h4>
      * <pre>
      *  // only copy the directory structure
      *  FileUtils.copyDirectory(srcDir, destDir, DirectoryFileFilter.DIRECTORY, false);
      *  </pre>
      *
-     * <h3>Example: Copy directories and txt files</h3>
+     * <h4>Example: Copy directories and txt files</h4>
      * <pre>
      *  // Create a filter for ".txt" files
      *  IOFileFilter txtSuffixFilter = FileFilterUtils.suffixFileFilter(".txt");

src/main/java/org/apache/commons/io/comparator/package.html

@@ -20,7 +20,7 @@
 <p>This package provides various {@link java.util.Comparator} implementations
 for {@link java.io.File}s.
 </p>
-<h3>Sorting</h3>
+<h2>Sorting</h2>
 <p>
   All the compartors include <i>convenience</i> utility <code>sort(File...)</code> and
   <code>sort(List)</code> methods.
@@ -39,7 +39,7 @@ <h3>Sorting</h3>
       File[] files = NameFileComparator.NAME_COMPARATOR.sort(dir.listFiles());
 </pre>
 
-<h3>Composite Comparator</h3>
+<h2>Composite Comparator</h2>
 <p>
   The <a href="CompositeFileComparator.html">CompositeFileComparator</a> can be used
   to compare (and sort lists or arrays of files) by combining a number other comparators.
@@ -57,7 +57,7 @@ <h3>Composite Comparator</h3>
       comparator.sort(files);
 </pre>
 
-<h3>Singleton Instances (thread-safe)</h3>
+<h2>Singleton Instances (thread-safe)</h2>
 <p>
  The {@link java.util.Comparator} implementations have some <i>convenience</i>
  singleton(<i>thread-safe</i>) instances ready to use:

src/main/java/org/apache/commons/io/input/BOMInputStream.java

@@ -43,7 +43,7 @@
  * </ul>
  *
  *
- * <h3>Example 1 - Detect and exclude a UTF-8 BOM</h3>
+ * <h2>Example 1 - Detect and exclude a UTF-8 BOM</h2>
  *
  * <pre>
  * BOMInputStream bomIn = new BOMInputStream(in);
@@ -52,7 +52,7 @@
  * }
  * </pre>
  *
- * <h3>Example 2 - Detect a UTF-8 BOM (but don't exclude it)</h3>
+ * <h2>Example 2 - Detect a UTF-8 BOM (but don't exclude it)</h2>
  *
  * <pre>
  * boolean include = true;
@@ -62,7 +62,7 @@
  * }
  * </pre>
  *
- * <h3>Example 3 - Detect Multiple BOMs</h3>
+ * <h2>Example 3 - Detect Multiple BOMs</h2>
  *
  * <pre>
  * BOMInputStream bomIn = new BOMInputStream(in,

src/main/java/org/apache/commons/io/monitor/FileEntry.java

@@ -30,7 +30,7 @@
  *   <li>Children - contents of a directory (see {@link File#listFiles(java.io.FileFilter)})</li>
  * </ul>
  *
- * <h3>Custom Implementations</h3>
+ * <h2>Custom Implementations</h2>
  * <p>
  * If the state of additional {@link File} attributes is required then create a custom
  * {@link FileEntry} with properties for those attributes. Override the