Correct typos in README.md.

![screen shot 2014-12-11 at 9 18 07 pm](https://cloud.githubusercontent.com/assets/5581107/5406040/98ad9afa-817b-11e4-88d8-d23c38d17921.png)
![screen shot 2014-12-11 at 9 18 19 pm](https://cloud.githubusercontent.com/assets/5581107/5406041/9f7f05f8-817b-11e4-807d-0f842017f00f.png)
![screen shot 2014-12-11 at 9 20 07 pm](https://cloud.githubusercontent.com/assets/5581107/5406046/a68a0d8e-817b-11e4-85c3-7f89c77d16fe.png)