Suggest "genders_for_select" for generating select options.

On my first read of this it suggested using `User.genders.keys` for a select, which isn't really the best option for a `select` since you need both the internal database value (either the Integer or String) **and** the **formatted** display value.

I changed my code to use the most excellent `User.genders_for_select` and it does exactly as you'd expect.