Galera after-merge fix

galera_sr.MDEV-28971: Adjust the expected error code.

This fixes up the merge commit 2b99e5f7

mysql-test/suite/galera_sr/t/MDEV-28971.test

@@ -15,6 +15,6 @@ INSERT INTO t1 VALUES ('AAF');
 SELECT SETVAL (SEQ, 100);
 ALTER TABLE t1 ADD CONSTRAINT constraint_1 UNIQUE (a);
 INSERT INTO t1 VALUES();
---error ER_KEY_COLUMN_DOES_NOT_EXITS
+--error ER_KEY_COLUMN_DOES_NOT_EXIST
 ALTER TABLE t1 ADD KEY(b (50));
 DROP TABLE t1,SEQ;