fix apiref CSS: overflowing text

In Firefox, when zoomed, some text overflows inside a floated container and is clipped (this also happens in Firefox when the font of code is changed).

![apiref-firefox](https://cloud.githubusercontent.com/assets/1437027/3286593/c1d04e80-f54b-11e3-81bc-fad644c987b9.png)

This commit removes the floating, since for a single child element it doesn't make sense anyway to be floated.

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I didn't change the CSS of `column` since it seems to be taken from AT.com directly, and it works fine on AT.com, but there the layout is different, it has a column and a sidebar.