Update Prefix Delegation section to use new config option
Update Prefix Delegation section to use new config option

Changes the configuration step in the IPv6 Prefix Delegation guide
to use the new configuration option introduced by:

https://review.openstack.org/#/c/230090

Change-Id: Id6c788800b6bf20250510accf9fa9324d8e736da
Depends-On: I43486c5a13ee2ff0097355afe7e1f3ef8794b185

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals