outdated link to whygitisbetterthanx

The page "Why Git is Better Than X" has moved from

    http://whygitisbetterthanx.com/

to

    http://z.github.io/whygitisbetter/

This should be reflected in the page

    http://doc.bazaar.canonical.com/migration/en/why-switch-to-bazaar.html

whose source is at

    http://doc.bazaar.canonical.com/migration/en/_sources/why-switch-to-bazaar.txt

by changing one line as follows:

- and `Why Git is Better Than X <http://whygitisbetterthanx.com/>`_.
+ and `Why Git is Better Than X <http://z.github.io/whygitisbetter/>`_.