[DOCS] Replace env.d with conf.d in provnet_groups docs
[DOCS] Replace env.d with conf.d in provnet_groups docs

This patch corrects a typo related to the provider network groups
documentation, replacing env.d with conf.d in the appropriate place.

Change-Id: I1b1432c0fccae7a6a1718f2849bb9f3c089c4703
Closes-Bug: #1829900

openstack/openstack-ansible,openstack/openstack-ansible,stackforge/os-ansible-deployment,stackforge/os-ansible-deployment