Improve partition_consumer docs formatting

Previously:

![screen shot 2014-12-13 at 21 36 35](https://cloud.githubusercontent.com/assets/9519/5426230/7e11aac2-8312-11e4-9878-eaa1a81a6e35.png)



This is what it looks like now:

![screen shot 2014-12-13 at 21 52 56](https://cloud.githubusercontent.com/assets/9519/5426229/76d61ea0-8312-11e4-980e-39676cdb0ce6.png)