Update _plugin_extending_campaigns.md

[kesc23]

documentation update as i ran into a problem trying to execute the code in the previous version of this example.
Now it implements Symfony\Component\EventDispatcher\EventSubscriberInterface instead of extending Mautic\CoreBundle\EventListener\CommonSubscriber as it does not exists in mautic anymore.

[RCheesley]

Thanks for making the PR @kesc23 - much appreciated!

We are actually in the process of moving the developer documentation over to Read The Docs in case that is of interest, and we are updating it in the process.

The new repo is here: https://github.com/mautic/developer-documentation-new - this is where we are trying to make all new changes. I think that this section might have already been ported over but you can check here: https://mautic-developer.readthedocs.io/en/latest