Improve contributing guide

This PR is about the YAML Front Matter example.

It contains two fixes:
* The example code was not being formatted properly and was being shown on one line.
* The example code was using `.html` in the permalink, whereas none of the posts actually do.

And one, imo, improvement of the example code, which makes it more relatable as it can be compared to the guide the reader is viewing at that very moment.

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### Before

![screen shot 2014-12-12 at 15 49 40](https://cloud.githubusercontent.com/assets/2320/5413407/82a4d908-8216-11e4-9f7d-d072f73d458a.png)

### After

![screen shot 2014-12-12 at 15 45 17](https://cloud.githubusercontent.com/assets/2320/5413399/671d11b4-8216-11e4-8417-3fab0598c854.png)