Move SerializableResource to ActiveModelSerializers namespace

Ref. https://github.com/rails-api/active_model_serializers/pull/1310

docs/howto/outside_controller_use.md

@@ -14,7 +14,7 @@ post = Post.create(title: "Sample post", body: "I love Active Model Serializers!
 options = {}
 
 # Create a serializable resource instance
-serializable_resource = ActiveModel::SerializableResource.new(post, options)
+serializable_resource = ActiveModelSerializers::SerializableResource.new(post, options)
 
 # Convert your resource into json
 model_json = serializable_resource.as_json
@@ -38,20 +38,20 @@ serializer = ActiveModel::Serializer.serializer_for(post, options)
 You could also retrieve the serializer via:
 
 ```ruby
-ActiveModel::SerializableResource.new(post, options).serializer
+ActiveModelSerializers::SerializableResource.new(post, options).serializer
 ```
 
 Both approaches will return an instance, if any, of the resource's serializer.
 
 ## Serializing before controller render
 
-At times, you might want to use a serializer without rendering it to the view. For those cases, you can create an instance of `ActiveModel::SerializableResource` with
+At times, you might want to use a serializer without rendering it to the view. For those cases, you can create an instance of `ActiveModelSerializers::SerializableResource` with
 the resource you want to be serialized and call `.as_json`.
 
 ```ruby
 def create
   message = current_user.messages.create!(message_params)
-  message_json = ActiveModel::SerializableResource.new(message).as_json
+  message_json = ActiveModelSerializers::SerializableResource.new(message).as_json
   MessageCreationWorker.perform(message_json)
   head 204
 end