drm/doc: Adapt GPU scheduler references for renamed C file

Fixes: "drm/scheduler: rename gpu_scheduler.c to sched_main.c"
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>