'challenge' list change

1-4 list under The Challenge section changed to match the other list style

Before:
![screen shot 2014-09-25 at 5 56 30 pm](https://cloud.githubusercontent.com/assets/3287583/4413611/6e6f890c-4507-11e4-85b1-c8783dcfc570.png)

After:
![screen shot 2014-09-25 at 5 57 37 pm](https://cloud.githubusercontent.com/assets/3287583/4413614/740da8bc-4507-11e4-8fd1-76160ab0d671.png)