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