REST API: Incomplete port mirroring API doc
REST API: Incomplete port mirroring API doc

Added missing "toPortId" to the "Mirror" resource.

Change-Id: I39761ec64158a5b8f5458f926193c4598b2d9927
Ref: https://midonet.atlassian.net/browse/MND-183
Signed-off-by: Jan Hilberath <14e793d896ddc8ca6911747228e86464cf420065@midokura.com>

midonet/midonet-docs,midonet/midonet-docs,midonet/midonet-docs,midonet/midonet-docs