raid1.c: fix a trivial typo in comments of freeze_array().

Cc: Jack Wang <jack.wang.usish@gmail.com>
Cc: Guoqing Jiang <gqjiang@suse.com>
Cc: John Stoffel <john@stoffel.org>
Acked-by: NColy Li <colyli@suse.de>
Signed-off-by: NZhilong Liu <zlliu@suse.com>
Signed-off-by: NShaohua Li <shli@fb.com>