Fix font-style for check-boxes, MODX 2.3.x

This PR fixes the following issue:

Before:
![screenshot_101714_092140_pm](https://cloud.githubusercontent.com/assets/114390/4682233/e0562c0a-561a-11e4-814c-b982da8f01f2.jpg)

After:
![screenshot_101714_092034_pm](https://cloud.githubusercontent.com/assets/114390/4682236/e70dfaaa-561a-11e4-9977-4d6797d62f21.jpg)