doc-rst: dev-overlay: fix the last warning

Fixes this warning:

Documentation/linux_tv/media/v4l/dev-overlay.rst:247: WARNING: Title underline too short.

struct v4l2_clip [4]_
----------------
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/linux_tv/media/v4l/dev-overlay.rst

@@ -244,7 +244,7 @@ exceeded are undefined. [3]_
 .. _v4l2-clip:
 
 struct v4l2_clip [4]_
-----------------
+---------------------
 
 ``struct v4l2_rect c``
     Coordinates of the clipping rectangle, relative to the top, left