Correct comment in index shard test

This commit fixes a comment in an index shard test which was inaccurate
after it was copied from another test and not modified to reflect the
reasoning in the test that it was copied into.

core/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -529,7 +529,7 @@ public class IndexShardTests extends IndexShardTestCase {
 
         /*
          * This operation completing means that the delay operation executed as part of increasing the primary term has completed and the
-         * gaps are filled.
+         * translog generation has rolled.
          */
         final CountDownLatch latch = new CountDownLatch(1);
         indexShard.acquirePrimaryOperationPermit(