Geo: Be consistent "primary" vs "primary node" in geo docs
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9634#note_152108622:
> We've been moving away from secondary node
, secondary nodes
, and primary node
in favor of **secondary**
, **secondaries**
, and **primary**
in the Geo docs.
- primary node
- secondary node (or secondary nodes)
We need to update a number of references.