fixing layout/style of disabled register button

Here's a tiny fix to improve the course about page for logged in users that I just noticed.

Before:
![before](https://f.cloud.github.com/assets/3364609/2069960/0ea8442c-8d16-11e3-937a-7d65abeda505.png)

After:
![after](https://f.cloud.github.com/assets/3364609/2069966/1259ef6c-8d16-11e3-973f-b2f5bf7ee130.png)