README: Update how to generate the docs
README: Update how to generate the docs

TrivialFix

Change-Id: Icbebe797f0b054d198550829193dde25fb84ad21

BiznetGIO/horizon,BiznetGIO/horizon,ChameleonCloud/horizon,noironetworks/horizon,BiznetGIO/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,openstack/horizon,NeCTAR-RC/horizon,openstack/horizon,NeCTAR-RC/horizon,noironetworks/horizon,openstack/horizon,yeming233/horizon,yeming233/horizon,noironetworks/horizon,NeCTAR-RC/horizon,BiznetGIO/horizon,noironetworks/horizon,ChameleonCloud/horizon,yeming233/horizon,yeming233/horizon,openstack/horizon,NeCTAR-RC/horizon