media: vivid.rst: loop_video is set on the capture devnode

The example on how to use and test Capture Overlay specified
the wrong video device node. Back in 2015 the loop_video control
moved from the output device to the capture device, but this
example code is still referring to the output video device.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>