Fix syntax highright on README.md

* before
![2013-07-15 18 08 28](https://f.cloud.github.com/assets/290782/796738/25fd64c8-ed2e-11e2-8646-c1aa997bd30c.png)

* after
![2013-07-15 18 08 34](https://f.cloud.github.com/assets/290782/796739/27284fac-ed2e-11e2-805e-bb5be00ce50a.png)