Fix typo in README.rst
Fix typo in README.rst

This is to fix a typo in README.rst

Change-Id: I31fbfc98994ca5137b1999f45beb94b345ae1b08

openstack/tempest,masayukig/tempest,cisco-openstack/tempest,masayukig/tempest,cisco-openstack/tempest,vedujoshi/tempest,Juniper/tempest,vedujoshi/tempest,openstack/tempest,Juniper/tempest