docs(Courses): Fix syntax issue in Developer Guide

The courses section should use commas between links to differentiate the instances of each link.  This improves the clarity of the docs by denoting that there are two separate links on each line.

![image](https://cloud.githubusercontent.com/assets/6476956/5426166/32049166-82f5-11e4-9e4e-3ffbbc65af70.png)