Install Guide: Fix screen/programlisting display
Install Guide: Fix screen/programlisting display

As Steve pointed out in https://review.openstack.org/#/c/53936/,
there are a couple of places where whitespace is wrongly used
for screen/programlisting. This fixes them.

Change-Id: I05b01498821188542e66d2517948475951597683
backport: havana

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