Fix URL to http://homotopytypetheory.org

The current URL http://www.homotopytypetheory.org is a
permanent redirect (301) to http://homotopytypetheory.org,
so the latter URL should be used instead.
Merge pull request #341 from vog/fix-url

Fix URL to http://homotopytypetheory.org (this is also what's used at other places in the book)