MINOR: Fix comment in quick union (apache#5244)

Reviewers: Bill Bejeck <[email protected]>, Matthias J. Sax <[email protected]>

streams/src/main/java/org/apache/kafka/streams/processor/internals/QuickUnion.java

@@ -42,7 +42,7 @@ public T root(T id) {
             throw new NoSuchElementException("id: " + id.toString());
 
         while (!parent.equals(current)) {
-            // do the path compression
+            // do the path splitting
             T grandparent = ids.get(parent);
             ids.put(current, grandparent);