Added clarification for Disqus comment setup

Updating the input field for the Disqus shortname to more closely match the Disqus registration page linked to in the helper text (http://disqus.com/admin/create/).

### Crowdhoster
![screen shot 2014-01-03 at 10 19 18 am](https://f.cloud.github.com/assets/1132438/1841366/9f5f5054-74a3-11e3-952e-3341c3169fe8.png)

### Disqus
![screen shot 2014-01-03 at 10 21 35 am](https://f.cloud.github.com/assets/1132438/1841374/e618c052-74a3-11e3-877f-8eca30c143b4.png)