Remove duplicate paragraph in authentication.rst

https://github.com/cakephp/docs/blob/3.0/fr/core-libraries/components/authentication.rst#cr%C3%A9er-des-classes-de-hash-de-mot-de-passe-personnalis%C3%A9
Merge pull request #1590 from romainlanz/patch-1

Remove duplicate paragraph in authentication.rst