minor #5537 Update design patter of Event Dispatcher (almacbe)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #5537).

Discussion
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Update design patter of Event Dispatcher

Commits
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ac46cbc Update event-dispatcher.rst

create_framework/event-dispatcher.rst

@@ -13,7 +13,7 @@ needs. Many software have a similar concept like Drupal or Wordpress. In some
 languages, there is even a standard like `WSGI`_ in Python or `Rack`_ in Ruby.
 
 As there is no standard for PHP, we are going to use a well-known design
-pattern, the *Observer*, to allow any kind of behaviors to be attached to our
+pattern, the *Mediator*, to allow any kind of behaviors to be attached to our
 framework; the Symfony EventDispatcher Component implements a lightweight
 version of this pattern: