media: docs-rst: v4l: Fix sink compose selection target documentation

The rectangle which the sink compose rectangle is related to is documented
to be the source compose bounds rectangle. This is in obvious conflict with
the ground rule of the format propagation (from sink to source). The reason
behind this is that this was always supposed to be the sink compose bounds
rectangle. Fix it.

Fixes: 955f645a ("[media] v4l: Add subdev selections documentation")

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>