Update README.rdoc to handle invalid single quote

In the README  **Sublime Text 2** part, when copy & paste the snippet code the single quote is invalid. By wrapping it in a `<tt>` tag can solve that.

![Screen Shot 2013-02-06 at 11 09 00 PM](https://f.cloud.github.com/assets/184520/131637/fd64a902-7066-11e2-907e-2f33e71aa06d.png)