Fixes#1163
As of 1d42d0d0ab the generator does not create an initializer.
This seems to have been unintentional, but it isn't actually required for PaperTrail to run correctly and there are [no plans to reinstate it](https://github.com/paper-trail-gem/paper_trail/issues/1163#issuecomment-435079777) so it's best to remove references to it from the documentation.
Also remove references to this in the generator description