media: Documentation: v4l: Fix ACTIVE route flag

The documentation in one occasion mentions the
VIDIOC_SUBDEV_STREAM_FL_ACTIVE flag. This was meant to be
V4L2_SUBDEV_STREAM_FL_ACTIVE as it's a flag, not an IOCTL. Fix it.

Fixes: cd2c7545 ("media: Documentation: Document S_ROUTING behaviour")
Reported-by: Samuel Wein PhD <sam@samwein.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>