media: docs: Fix vimc default pipeline graph

RGB/YUV Input is sensor type and it should be sub-dev node.
To generate this dot graph

sudo modprobe vimc
media-ctl --print-dot

Signed-off-by: Kwang Son <dev.kwang.son@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>