[contributor guide] trivial fix
[contributor guide] trivial fix

This patch removes the superfluous 'that' from the
Installation Tutorial testing chapter of the Release article.

Change-Id: I710e5d2a19b2cf433c00a2244989eaa11cacfa94

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals