Fix errors in commit a04bb65f.

Not a lot of commentary needed here really.

src/backend/commands/async.c

@@ -1421,7 +1421,7 @@ pg_notification_queue_usage(PG_FUNCTION_ARGS)
 /*
  * Return the fraction of the queue that is currently occupied.
  *
- * The caller must hold AysncQueueLock in (at least) shared mode.
+ * The caller must hold AsyncQueueLock in (at least) shared mode.
  */
 static double
 asyncQueueUsage(void)

src/test/isolation/isolation_schedule

@@ -41,4 +41,5 @@ test: alter-table-1
 test: alter-table-2
 test: alter-table-3
 test: create-trigger
+test: async-notify
 test: timeouts