block: fix a typo in comment of BLK_MQ_POLL_STATS_BKTS

Update comment typo _consisitent_ to _consistent_ from following commit.
commit 0206319f ("blk-mq: Fix poll_stat for new size-based bucketing.")

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>