io_uring: documentation fixup

Fix incorrect name reference in comment. ki_filp does not exist in the
struct, but file does.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220224105157.1332353-1-dylany@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>