Fix deprecated syntax in old controller helper

This syntax for rendering a file isn't valid in Rails 6.x...

lib/spree/core/controller_helpers/common.rb

@@ -42,7 +42,7 @@ module Spree
             respond_to do |type|
               type.html {
                 render status: :not_found,
-                       file: "#{::Rails.root}/public/404",
+                       file: Rails.root.join("public/404.html"),
                        formats: [:html],
                        layout: nil
               }