UBI: fix typo in a message

When a PEB passes the torture test, UBI prints
"do not mark it a bad", but should print "do not mark it as bad".
This patch corrects the typo.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/io.c

@@ -465,7 +465,7 @@ static int torture_peb(struct ubi_device *ubi, int pnum)
 	}
 
 	err = patt_count;
-	ubi_msg("PEB %d passed torture test, do not mark it a bad", pnum);
+	ubi_msg("PEB %d passed torture test, do not mark it as bad", pnum);
 
 out:
 	mutex_unlock(&ubi->buf_mutex);