Fixed ~git beeing displayed as link to issues tagged with git in architecture.md

The architecture documentation explains `~git` from the picture below which is replaced by a [link to issues tagged with `git`](https://gitlab.com/gitlab-org/gitlab-ce/issues?label_name=git). I believe that this was unintended and should be fixed by using backticks.

![Screenshot_from_2016-02-13_21-36-30](/uploads/5ea1a60e8b230952874e58eab0abe1ac/Screenshot_from_2016-02-13_21-36-30.png)

See merge request !2814