Tiny improvement of README 0-9-stable

Really small thing, but due to lack of commas github is adding extra red backgrounds in "Embedding Polymorphic Associations" examples, making it look like some kind of errors.
![screen shot 2014-12-09 at 16 09 08](https://cloud.githubusercontent.com/assets/68907/5359475/fa7e0b58-7fbd-11e4-8d3a-e5ad6267c2a3.png)