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>

Documentation/admin-guide/media/vivid.rst

@@ -1318,7 +1318,7 @@ instance. This setup would require the following commands:
 	$ v4l2-ctl -d2 -i2
 	$ v4l2-ctl -d2 -c horizontal_movement=4
 	$ v4l2-ctl -d1 --overlay=1
-	$ v4l2-ctl -d1 -c loop_video=1
+	$ v4l2-ctl -d0 -c loop_video=1
 	$ v4l2-ctl -d2 --stream-mmap --overlay=1
 
 And from another console: