drm/doc: fix underline for section "DRM Sync Objects"

It doesn't line up.
Signed-off-by: Simon Ser <contact@emersion.fr>
Suggested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230803102505.392577-1-contact@emersion.fr

Documentation/gpu/drm-mm.rst

@@ -520,7 +520,7 @@ DRM Cache Handling and Fast WC memcpy()
 .. _drm_sync_objects:
 
 DRM Sync Objects
-===========================
+================
 
 .. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
    :doc: Overview