Change confirm step label to uniform it with other steps

It's a little one but I think it's better to uniform the way we call steps name label between each checkout steps.

Into other steps it's used a separate label, for example into [delivery](https://github.com/spree/spree/blob/master/frontend/app/views/spree/checkout/_delivery.html.erb#L2) and [payment](https://github.com/spree/spree/blob/master/frontend/app/views/spree/checkout/_payment.html.erb#L2).

Also we have a proper [I18n label](https://github.com/spree/spree/blob/master/core/config/locales/en.yml#L431) for this, so why do not use it?