block: Correct a documentation comment in blk-cgroup.c

block: Correct a documentation comment in blk-cgroup.c

Commit 99e603874366
("blk-cgroup: pass a gendisk to the blkg allocation helpers") changed
blkg_alloc() to take a struct gendisk instead of a struct request_queue,
but the documentation comment still referred to q.

So, update that comment to refer to disk instead and fix a typo.

Signed-off-by: Nicky Chorley <ndchorley@gmail.com>
Link: https://lore.kernel.org/r/20240114191056.6992-1-ndchorley@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>