Remove "Save" label from submit button in form partial

The [erb template](https://github.com/rails/rails/blob/master/railties/lib/rails/generators/erb/scaffold/templates/_form.html.erb) doesn't explicitly set the text on the button as it can be generated automatically and [changes depending if the resource is new or existing]((http://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-submit) (a bit like the url in `form_for`).