pg_createsubscriber: Fix error complaining about the wrong thing

The code updates the system identifier, then runs pg_walreset; if the
latter fails, it complains about the former, which makes no sense.
Change the error message to complain about the right thing.

Noticed while reviewing a patch touching nearby code.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Backpatch-through: 17

src/bin/pg_basebackup/pg_createsubscriber.c

@@ -706,9 +706,9 @@ modify_subscriber_sysid(const struct CreateSubscriberOptions *opt)
 		int			rc = system(cmd_str);
 
 		if (rc == 0)
-			pg_log_info("subscriber successfully changed the system identifier");
+			pg_log_info("subscriber successfully reset WAL on the subscriber");
 		else
-			pg_fatal("could not change system identifier of subscriber: %s", wait_result_to_str(rc));
+			pg_fatal("could not reset WAL on subscriber: %s", wait_result_to_str(rc));
 	}
 
 	pg_free(cf);