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
(cherry picked from commit f190f69d9fb1b52396d736c656f487368d4e3692)
Merge "Install Guide: Fix screen/programlisting display" into stable/havana