Update code listing to use monospace font

Before:
![image](https://cloud.githubusercontent.com/assets/1488921/5164949/473a406a-73de-11e4-9793-9ff7e821a960.png)

After:
![image](https://cloud.githubusercontent.com/assets/1488921/5165157/4d759fd0-73e1-11e4-8336-04250e278935.png)