Improve the instructions on the add node admin page for GEO

Screen_Shot_2017-08-30_at_6.41.59_PM

We should probably change the wording here as this is the public key for the machine located at /var/opt/gitlab/.ssh/id_rsa.pub for the node you are adding. Not that of a "gitlab user". Also maybe we should link to https://docs.gitlab.com/ee/gitlab-geo/configuration.html#step-5-enabling-the-secondary-gitlab-node instead of https://prod.geo.gitlab.com/help/ssh/README ?