Documentation Fix: Remove broken link on rails 3 guide

As raised in https://github.com/rails/rails/issues/12300

I have simply removed the link as I could not find a cached copy.  I did try Google cache at http://webcache.googleusercontent.com/search?q=cache:http://omgbloglol.com/post/353978923/the-path-to-rails-3-approaching-the-upgrade but it was not found.