md: fix a trivial typo in comments

Fix a trivial typo in md_ioctl().
Signed-off-by: NWei Fang <fangwei1@huawei.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/md.c

@@ -6883,7 +6883,7 @@ static int md_ioctl(struct block_device *bdev, fmode_t mode,
 
 	case ADD_NEW_DISK:
 		/* We can support ADD_NEW_DISK on read-only arrays
-		 * on if we are re-adding a preexisting device.
+		 * only if we are re-adding a preexisting device.
 		 * So require mddev->pers and MD_DISK_SYNC.
 		 */
 		if (mddev->pers) {