1. change path to destination_path, 2. add trailing slash to url

Somewhat described here:
[http://middlemanapp.com/advanced/sitemap/](http://middlemanapp.com/advanced/sitemap/) 

From the official site:
[https://github.com/middleman/middleman-guides/blob/master/source/sitemap.xml.builder](https://github.com/middleman/middleman-guides/blob/master/source/sitemap.xml.builder)