Fix indent for sample code on '/understanding-ember/debugging'

The indent size equal 2 is better than 4.

Before:
![2014-12-15 4 08 25](https://cloud.githubusercontent.com/assets/290782/5428681/7325824c-8410-11e4-8f8c-6151c412d59b.png)

After:
![2014-12-15 4 08 59](https://cloud.githubusercontent.com/assets/290782/5428683/76b70c8c-8410-11e4-99a4-8c9697d4906d.png)