Update service.rb (#42766)

* Update service.rb

The documentation for using the Active Storage from outside of the Ruby on Rails application is not according to the implementation. According to the implementation, we are looking for the env name instead of the `service` name in the configuration which is a hash( same as loading a `storage.yml`)

* Update service.rb

* Update service.rb

activestorage/lib/active_storage/service.rb

@@ -35,8 +35,8 @@ module ActiveStorage
   # can configure the service to use like this:
   #
   #   ActiveStorage::Blob.service = ActiveStorage::Service.configure(
-  #     :Disk,
-  #     root: Pathname("/foo/bar/storage")
+  #     :local,
+  #     { local: {service: "Disk",  root: Pathname("/tmp/foo/storage") } }
   #   )
   class Service
     extend ActiveSupport::Autoload