[media] doc: kapi: fix typo

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/v4l2-core.rst

-Video2Linux devices
+Video4Linux devices
 -------------------
 
 .. toctree::