media: media-ioc-g-topology.rst: fix interface-to-entity link description

The source_id and sink_id descriptions were the same for interface-to-entity
links. The source_id is the interface ID, not the entity ID. Fix this.

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