Format "protocol versions" table content to improve readability

I thought this could improve readability a little while not adding too much HTML.

Current version: http://dg0.dtrt.org/en/developer-reference#protocol-versions

Screenshot:

![capture du 2014-11-14 20 01 38](https://cloud.githubusercontent.com/assets/3578089/5055881/bb4b19b8-6c39-11e4-8000-35d19014d829.png)