io_uring: Fix comment of io_get_sqe

The sqe_ptr argument has been gone since 709b302f

 (io_uring:
simplify io_get_sqring, 2020-04-08), made the return value of the
function. Update the comment accordingly.

Signed-off-by: Fam Zheng <fam.zheng@bytedance.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20210604164256.12242-1-fam.zheng@bytedance.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>