Replace an occurrence of slave with standby

Commit a1ef920e27 replaced the use of
slave with standby, but overlooked this comment.

Author: Daniel Gustafsson

contrib/pg_stat_statements/pg_stat_statements.c

@@ -625,7 +625,7 @@ pgss_shmem_startup(void)
 
 	/*
 	 * Remove the persisted stats file so it's not included in
-	 * backups/replication slaves, etc.  A new file will be written on next
+	 * backups/replication standbys, etc.  A new file will be written on next
 	 * shutdown.
 	 *
 	 * Note: it's okay if the PGSS_TEXT_FILE is included in a basebackup,