IO-886 Replaced incorrect regular expression replaceAll with plain replace (#828)

Author: peterdemaeyer

src/main/java/org/apache/commons/io/file/PathUtils.java

@@ -139,7 +139,7 @@ private static boolean equalsIgnoreFileSystem(final Path path1, final Path path2
 
         static String extractKey(final String separator, final String string) {
             // Replace the file separator in a path string with a string that is not legal in a path on Windows, Linux, and macOS.
-            return string.replaceAll("\\" + separator, ">");
+            return string.replace(separator, ">");
         }
 
         final boolean equals;