Fix some System.import docs format issues.

- Fix typo (backtip instead of single quote)
- Use consistenly singles quotes through code examples
- Use consistent indentation for `script` tags examples

Before: 
![screen shot 2014-11-11 at 16 56 46](https://cloud.githubusercontent.com/assets/724877/4999759/ec48a8be-69c3-11e4-8131-b14c8e258f7b.png)
![screen shot 2014-11-11 at 16 57 01](https://cloud.githubusercontent.com/assets/724877/4999767/f0796e28-69c3-11e4-897d-daed09eab828.png)
![screen shot 2014-11-11 at 16 57 13](https://cloud.githubusercontent.com/assets/724877/4999769/f33a7364-69c3-11e4-808f-9ecea971fddf.png)
![screen shot 2014-11-11 at 16 57 21](https://cloud.githubusercontent.com/assets/724877/4999772/f8f54950-69c3-11e4-99c2-9b9ad4997efd.png)

After:
![screen shot 2014-11-11 at 16 58 52](https://cloud.githubusercontent.com/assets/724877/4999788/1e6969e6-69c4-11e4-92d1-e808366f65dc.png)
![screen shot 2014-11-11 at 16 59 02](https://cloud.githubusercontent.com/assets/724877/4999789/2156e098-69c4-11e4-9d8a-aa3fc04a1b0d.png)
![screen shot 2014-11-11 at 16 59 13](https://cloud.githubusercontent.com/assets/724877/4999790/24ab97e8-69c4-11e4-81bb-230c133eba3e.png)
![screen shot 2014-11-11 at 16 59 20](https://cloud.githubusercontent.com/assets/724877/4999791/27feef76-69c4-11e4-96b7-245f90941e20.png)