Add missing command for pg_maintain in comment

The comment in pg_class_aclmask_ext() which lists the allowed commands
for the pg_maintain role lacked LOCK TABLE.

Reported-by: Yusuke Sugie <btsugieyuusuke@oss.nttdata.com>
Reviewed-by: Yugo Nagata <nagata@sraoss.co.jp>
Discussion: https://postgr.es/m/034d3c60f5daba1919cd90f236b2e22d@oss.nttdata.com

src/backend/catalog/aclchk.c

@@ -3449,7 +3449,7 @@ pg_class_aclmask_ext(Oid table_oid, Oid roleid, AclMode mask,
 	 * Check if ACL_MAINTAIN is being checked and, if so, and not already set
 	 * as part of the result, then check if the user is a member of the
 	 * pg_maintain role, which allows VACUUM, ANALYZE, CLUSTER, REFRESH
-	 * MATERIALIZED VIEW, and REINDEX on all relations.
+	 * MATERIALIZED VIEW, REINDEX, and LOCK TABLE on all relations.
 	 */
 	if (mask & ACL_MAINTAIN &&
 		!(result & ACL_MAINTAIN) &&