media: cec-core: fix a Sphinx warning

/devel/v4l/patchwork/Documentation/media/kapi/cec-core.rst:369: WARNING: Title underline too short.

CEC Notifier framework
-----------------
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/cec-core.rst

@@ -366,7 +366,7 @@ CEC pin transitions from low to high or vice versa.
 .. kernel-doc:: include/media/cec-pin.h
 
 CEC Notifier framework
------------------
+----------------------
 
 Most drm HDMI implementations have an integrated CEC implementation and no
 notifier support is needed. But some have independent CEC implementations