C comment: correct heading of extension query

Reported-by: Justin Pryzby

Discussion: https://postgr.es/m/20210803161345.GZ12533@telsasoft.com

Backpatch-through: 9.6

src/bin/pg_upgrade/version.c

@@ -494,7 +494,7 @@ report_extension_updates(ClusterInfo *cluster)
 		DbInfo	   *active_db = &cluster->dbarr.dbs[dbnum];
 		PGconn	   *conn = connectToServer(cluster, active_db->db_name);
 
-		/* find hash indexes */
+		/* find extensions needing updates */
 		res = executeQueryOrDie(conn,
 								"SELECT name "
 								"FROM pg_available_extensions "