blk-cgroup: Fix doc related to blkcg_exit_queue

Since 4cf6324b, a portion of function blk_cleanup_queue was moved to
a newly created function called blk_exit_queue, including the call of
blkcg_exit_queue. So, adjust the documenation according.
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -1269,7 +1269,7 @@ void blkcg_drain_queue(struct request_queue *q)
  * blkcg_exit_queue - exit and release blkcg part of request_queue
  * @q: request_queue being released
  *
- * Called from blk_release_queue().  Responsible for exiting blkcg part.
+ * Called from blk_exit_queue().  Responsible for exiting blkcg part.
  */
 void blkcg_exit_queue(struct request_queue *q)
 {