Fix Downloads table layout

The fixes the layout of the first table in the [Downloads](http://git-scm.com/downloads) page.

Before:

![screen shot 2014-01-31 at 12 34 13](https://f.cloud.github.com/assets/65057/2050000/a5b11ff4-8a6b-11e3-9caf-81d169980464.png)

After:

![screen shot 2014-01-31 at 12 34 45](https://f.cloud.github.com/assets/65057/2050010/baa23fc4-8a6b-11e3-8f03-75252bab4c77.png)

/cc @jasonlong