Add set-nth to the list functions group in the function documentation

This PR updates the functions reference documentation by adding `set-nth` to the list functions table.

![screen shot 2014-10-29 at 3 28 02 pm](https://cloud.githubusercontent.com/assets/579928/4821091/00bf98de-5f24-11e4-86d3-dd1d752836cc.png)

Yep, done. https://github.com/sass/sass/pull/1501