block, bfq: update outdated comment

The function bfq_bfqq_may_idle() was renamed as bfq_better_to_idle()
in commit 277a4a9b56cd ("block, bfq: give a better name to
bfq_bfqq_may_idle").  Update the comment accordingly.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Jens Axboe <axboe@kernel.dk>