Fixes reference to __super__ property on the generated docs

I was let down by the docs saying that a **super** property is available on the extending generator, when the actual property name is **\_\_super__**.

Below is a screenshot from the offending **Base** page and [here is the link to it](http://yeoman.github.io/generator/Base.html).

![image demonstrating the problem](http://i.imgur.com/6mf1RZS.png)

Turns out it was **jsdoc** parsing the underscores of the \_\_super__ property as markdown.

This PR fixes it :)