Broken link in developers documentation

In `docs/contributing/hands-on.md`, on line 18 the link is broken due to the double brackets in the markdown.

```
New contributors are recommended to start by implementing an [already approved hands-on proposal]((https://github.com/git-mastery/exercises/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22hands-on%20discussion%22%20label%3A%22help%20wanted%22)).
```
