Fix typo in comment in ReplicationOperation.java

Within two lines of each other appears "fallthrough" and "fall through",
both typed by the same person who should have been paying better
attention and only one of these is correct and the inconsistency is
bothersome. This commit fixes the errant one.

Author: jasontedor

core/src/main/java/org/elasticsearch/action/support/replication/ReplicationOperation.java

@@ -189,7 +189,7 @@ public void onResponse(ReplicaResponse response) {
                 try {
                     primary.updateLocalCheckpointForShard(response.allocationId(), response.localCheckpoint());
                 } catch (final AlreadyClosedException e) {
-                    // okay, the index was deleted or this shard was never activated after a relocation; fallthrough and finish normally
+                    // okay, the index was deleted or this shard was never activated after a relocation; fall through and finish normally
                 } catch (final Exception e) {
                     // fail the primary but fall through and let the rest of operation processing complete
                     final String message = String.format(Locale.ROOT, "primary failed updating local checkpoint for replica %s", shard);