io_uring: fix typo in io_uring.h comment

Just a basic s/thig/this swap, fixing up a typo introduced by a commit
added in the 6.1 release.

Fixes: 9cda70f6

 ("io_uring: introduce fixed buffer support for io_uring_cmd")
Signed-off-by: Jens Axboe <axboe@kernel.dk>