Modify the type of subnet in the api-ref
Modify the type of subnet in the api-ref

The type of parameter "subnet-obj" should be object, not string.

Change-Id: Ied9802df1f63a59f4af6850bb1d863f5b70ed12f

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib