Site /feed.xml is RSS 2.0, fix auto-discovery header

Fixes a small issue reported on #2996, where the RSS auto-discovery header in a generated site is linked as `type="application/atom+xml"` where the `/feed.xml` file is in fact in RSS 2.0 format and should be `type="application/rss+xml"`. 

The page http://technotes.iangreenleaf.com/posts/2013-10-17-rss-the-least-wrong-way.html seems like a neat reference to double-check these values.