There is an extra `)` at the end of this URL. Also, Markdown reference IDs don't have to look like variable names; let's do this instead:

```
This cookbook provides a `machine_tag` resource that can create, delete, list, and
search [machine tags][Tagging] in the Vagrant and RightScale environments.
```
`[Tagging]: http://support.rightscale.com/12-Guides/RightScale_101/06-Advanced_Concepts/Tagging`