Guides: Update serialization section [ci skip]

The current guidance on how to initialize a custom serializer does not
work. Instead, it raises a `NameError`.

I found I needed to use the `to_prepare` [initialization events][]
to correctly configure a custom serializer:

Additionally, add the private `klass` instance method to match the
example in the API.

[initialization events]: https://guides.rubyonrails.org/configuring.html#initialization-events

Author: stevepolitodesign

guides/source/active_job_basics.md

@@ -600,11 +600,6 @@ You can extend the list of supported argument types. You just need to define you
 ```ruby
 # app/serializers/money_serializer.rb
 class MoneySerializer < ActiveJob::Serializers::ObjectSerializer
-  # Checks if an argument should be serialized by this serializer.
-  def serialize?(argument)
-    argument.is_a? Money
-  end
-
   # Converts an object to a simpler representative using supported object types.
   # The recommended representative is a Hash with a specific key. Keys can be of basic types only.
   # You should call `super` to add the custom serializer type to the hash.
@@ -619,6 +614,12 @@ class MoneySerializer < ActiveJob::Serializers::ObjectSerializer
   def deserialize(hash)
     Money.new(hash["amount"], hash["currency"])
   end
+
+  private
+    # Checks if an argument should be serialized by this serializer.
+    def klass
+      Money
+    end
 end
 ```
 
@@ -636,7 +637,7 @@ to set-up serializers to be loaded only once, e.g. by amending `config/applicati
 # config/application.rb
 module YourApp
   class Application < Rails::Application
-    config.autoload_once_paths << Rails.root.join('app', 'serializers')
+    config.autoload_once_paths << "#{root}/app/serializers"
   end
 end
 ```