Install Guide: Fix plug-in in neutron code
Install Guide: Fix plug-in in neutron code

Revert some s/plugin/plug-in/ done as part of change
https://review.openstack.org/#/c/53051/

The code changes were all wrong and have been fixed.

Change-Id: I10b151d2c7a17a1c67639f402cedeeef107c703d
Backport: havana

saeki-masaki/openstack-manuals,openstack/openstack-manuals,potsmaster/openstack-manuals,openstack/openstack-manuals,saeki-masaki/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,gongysh2004/openstack-manual,Akasurde/openstack-manuals,openstack/openstack-manuals,kairoaraujo/openstack-manuals,potsmaster/openstack-manuals,gongysh2004/openstack-manual