docs: Update Freenode to OFTC
docs: Update Freenode to OFTC

see: http://lists.openstack.org/pipermail/openstack-discuss/2021-May/022718.html
Change-Id: Ib8f10878ceb1ae222fffa1e9e29ea13cc019e9f6

openstack/vitrage,openstack/vitrage,openstack/vitrage