drm/radeon: fix typo in cik_ring_ib_execute documentation (v2)

v2: agd: clarify commit message, fix "an" as spotted by Michel.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>