drm/docs: Fix typo in __drm_atomic_helper_connector_reset

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-4-maarten.lankhorst@linux.intel.com