Corrected bundle command in Contribution.md

Corrected this

![download 4](https://f.cloud.github.com/assets/63040/817645/bd24e9fe-ef7a-11e2-8adb-8ec19346b124.png)

too this 

![download 5](https://f.cloud.github.com/assets/63040/817646/c384c918-ef7a-11e2-8a9a-41ef9f52a314.png)