Fix Zeitwerk error when Pathname added to paths

The previous example added a Pathname to `autoload_once_paths`, which
raises an error on boot:

```
loader wants to manage directory /home/hartley/test/eight/app/serializers, which is already managed by loader
```

so the example is updated to match the one in Autoloading and Reloading
Constants.

Author: skipkayhil

guides/source/active_job_basics.md

@@ -636,7 +636,7 @@ 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
 ```