Fix password validation example in `Model.extend` docs
Fix password validation example in `Model.extend` docs

Fixes #854

cspotcode/bookshelf,cspotcode/bookshelf,seegno-forks/bookshelf,jadengore/bookshelf,rbong/bookshelf,patbenatar/bookshelf,rbong/bookshelf,apla/bookshelf,seegno-forks/bookshelf,TinOo512/bookshelf,tgriesser/bookshelf,chrisbroome/bookshelf,tgriesser/bookshelf,jadengore/bookshelf,apla/bookshelf,chrisbroome/bookshelf,bookshelf/bookshelf,TinOo512/bookshelf,VideoAmp/bookshelf,patbenatar/bookshelf,jadengore/bookshelf,jordansexton/bookshelf,VideoAmp/bookshelf,patbenatar/bookshelf,jordansexton/bookshelf,bookshelf/bookshelf,VideoAmp/bookshelf