French translation correction 

I followed the French CakePHP tutorial in order to create a blog. In Login and Authentication Parts, I found a problem. When I added a user in th database and later when I wished to log in with it, an error was returned to "You are not authorized to access That location". The problem was that the conventions of CakePHP were waiting tables to contain the fileds "username" and "password" for a user

This commit is here to correct those mistakes