Rename Gitaly nodes in Praefect architecture diagram

Problem to solve

The Praefect architecture diagram has three nodes that are titled Praefect-Git-1/2/3 leading users to believe these may be Praefect nodes if they just look at the diagram.

Proposal

Rename nodes behind Praefect to make them clear they are just Gitaly nodes.

Who can address the issue

groupgitaly team