fix page alignment in some browsers (firefox)

In Firefox, it seems that the page is left aligned:
![screenshot from 2013-12-05 14 59 32](https://f.cloud.github.com/assets/1175095/1688086/73091828-5e00-11e3-889a-725cff59bb32.png)

By setting the horizontal margin to `auto`, that fixes it so that it's centered, consistent with how Chrome renders the page:
![screenshot from 2013-12-05 14 59 46](https://f.cloud.github.com/assets/1175095/1688091/93062fe4-5e00-11e3-9c78-32f755b64f1f.png)