blk-mq: Fix a comment in include/linux/blk-mq.h

The 'hctx_list' member of struct blk_mq_hw_ctx is not a list head but
instead an entry in q->unused_hctx_list. Fix the comment above this
struct member.

Fixes: d386732b ("blk-mq: fill header with kernel-doc")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Cc: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>