[media] v4l-drivers/fourcc.rst: fix typo

Linux4Linux -> Video4Linux
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/v4l-drivers/fourcc.rst

-Guidelines for Linux4Linux pixel format 4CCs
+Guidelines for Video4Linux pixel format 4CCs
 ============================================
 
 Guidelines for Video4Linux 4CC codes defined using v4l2_fourcc() are