Fix about overlay

Just noticed this (in Firefox & Chrome): If you click on the "About" link in the navigation, the appearing text should read something like "[...] This site uses YARD to ...", but what it actually reads is:

![rubydoc-info](https://cloud.githubusercontent.com/assets/311914/2665122/87ffa7a6-c089-11e3-9c42-9a30587c4057.png)

This PR is fixing the corresponding HTML.