Install Guide: One more Debian config fix

I missed one place in patch https://review.openstack.org/#/c/55090/.

Change-Id: I7d48796f91d1c90f048dd036f40d1c60f1f4d58c
backport: havana
(cherry picked from commit d722eef6f19984cdb0cb14a8524c84f4bd2307c4)