`contributing.md`: Fix link

The `contributing.md` file contained a markdown-flavored link inside of embedded html, which resulted in this:

![screenshot_2014-10-20_21_51_08_png_-_dropbox](https://cloud.githubusercontent.com/assets/6025224/4707994/18bdb21e-5893-11e4-8e64-3ed02d453f33.png)


If you want to link something up in embedded html, you need to just use `<a>`.