[networking] Correct enable_dhcp description in IPv6 section
[networking] Correct enable_dhcp description in IPv6 section

enable_dhcp is not a config option in neutron.conf.
It is an attribute of neutron subnet object.

Change-Id: Ib6367981b6f91a29d6cda1d6c2e9f915890a7b04

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