Add missing Javadoc tag.

Author: Gary Gregory

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

@@ -260,7 +260,8 @@
  *    }
  *  }
  * </pre>
- *
+ * 
+ * @param <T> The result type, like {@link File}.
  * @since 1.3
  *
  */