Change views from class="fields" to class="field" to be same as rails scaffolding

From `class="fields"` to `class="field"`

This is how rails scaffolding does it: https://github.com/rails/rails/blob/08754f12e65a9ec79633a605e986d0f1ffa4b251/railties/lib/rails/generators/erb/scaffold/templates/_form.html.erb

This is a part of https://github.com/plataformatec/devise/issues/3256 and fixes https://github.com/plataformatec/devise/pull/3266.

Thanks!