[OVN][DOC] Update path to L3RouterPlugin
[OVN][DOC] Update path to L3RouterPlugin

The path was old pointing not existing networking-ovn path.

Fixed also first paragraph about OVS/OVN installation.

Change-Id: Iee77443c0ac8cb83d4e380c53db8405eca642b2b
Closes-Bug: #1879747

mahak/neutron,openstack/neutron,mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron