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>

Documentation/gpu/drm-mm.rst

@@ -505,7 +505,7 @@ GPU Scheduler
 Overview
 --------
 
-.. kernel-doc:: drivers/gpu/drm/scheduler/gpu_scheduler.c
+.. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c
    :doc: Overview
 
 Scheduler Function References
@@ -514,5 +514,5 @@ Scheduler Function References
 .. kernel-doc:: include/drm/gpu_scheduler.h
    :internal:
 
-.. kernel-doc:: drivers/gpu/drm/scheduler/gpu_scheduler.c
+.. kernel-doc:: drivers/gpu/drm/scheduler/sched_main.c
    :export: