Fix to highlight code in document

Fix to highlight `{{textarea}}`.

* before
![2013-12-19 4 11 35](https://f.cloud.github.com/assets/290782/1776569/98342134-6818-11e3-8c3c-1e945d2a5b44.png)

* after
![2013-12-19 4 12 17](https://f.cloud.github.com/assets/290782/1776570/98364c70-6818-11e3-80c6-98d5c423bccb.png)