Use correct class for chosen in admin CSS

The dropdowns all use .chosen-container but our CSS was still using .chzn-container from a previous version. Here's what it looks like before and after:

![chzn-margins](https://f.cloud.github.com/assets/74812/1674408/e389c236-5cf8-11e3-87e8-1f8072d26252.png)