Update repo URL in installation instructions

I got a "Repository not found" error when attempting to follow these installation instructions.

```
[10:12:28] ../opensource $ git clone git://github.com/tracksapp/tracks.git
Cloning into 'tracks'...
fatal: remote error:
  Repository not found.
[10:12:34] ../opensource $ git clone git@github.com:TracksApp/tracks.git
Cloning into 'tracks'...
remote: Counting objects: 40754, done.
remote: Compressing objects: 100% (12445/12445), done.
remote: Total 40754 (delta 27328), reused 39791 (delta 26547)
Receiving objects: 100% (40754/40754), 16.98 MiB | 1.90 MiB/s, done.
Resolving deltas: 100% (27328/27328), done.
```