media: docs: uAPI: Fix documentation of 'which' field for routing ioctls

The routing ioctls documentation incorrectly describes the 'which'
field, due to a bad copy & paste. Fix it.

Fixes: ea73eda5 ("media: Documentation: Add GS_ROUTING documentation")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>