[DDF] "Reference documentation" link should link to 2.8 reference

"Reference documentation" link should link to 2.8 reference. The current url is https://github.com/3scale/3scale-operator/blob/3scale-2.7.0-ER2/doc/apimanager-reference.md

Reported by: rhn-support-tkonishi

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.8/html/installing_3scale/install-threescale-on-openshift-guide#annotations:a2331ef4-e6ed-4816-b171-923cdf33c8a9