Update example code for obtaining a locked DirCache

Previous example was calling a method that no longer exists
on DirCache.

Change-Id: I864842c58df4f116b318dc4a150b789745769a0d

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheBuildIterator.java

@@ -66,7 +66,7 @@ import org.eclipse.jgit.treewalk.AbstractTreeIterator;
  * algorithm. For example deleting a path:
  *
  * <pre>
- * final DirCache dirc = DirCache.lock(db);
+ * final DirCache dirc = db.lockDirCache();
  * final DirCacheBuilder edit = dirc.builder();
  *
  * final TreeWalk walk = new TreeWalk(db);