docs/ardana/testing: pluralise prerequisites
docs/ardana/testing: pluralise prerequisites

There is more than one prerequisite.

aspiers/automation,gosipyan/automation,aspiers/automation,SUSE-Cloud/automation,SUSE-Cloud/automation,SUSE-Cloud/automation,gosipyan/automation,gosipyan/automation,aspiers/automation,SUSE-Cloud/automation,gosipyan/automation,aspiers/automation