Fix typo in IndexDiff.getModified() comment.

Removes an extra instance of the word 'on'.

Change-Id: Ie5f137f0dda440f5879f6d5c62ebce0431530ad7
Signed-off-by: Kevin Sawicki <kevin@github.com>

org.eclipse.jgit/src/org/eclipse/jgit/lib/IndexDiff.java

@@ -377,7 +377,7 @@ public class IndexDiff {
 	}
 
 	/**
-	 * @return list of files on modified on disk relative to the index
+	 * @return list of files modified on disk relative to the index
 	 */
 	public Set<String> getModified() {
 		return modified;