minor #9912 Observer instead of Mediator pattern (ruff3d, javiereguiluz)

This PR was submitted for the 2.7 branch but it was merged into the 2.8 branch instead (closes #9912).

Discussion
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Observer instead of Mediator pattern

Actually Event Dispatcher component implements Observer pattern (GoF) but not a Mediator.
I think we should change this description so as not to confuse users in the future.

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Commits
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21ed3d2 Mention that the component implements both patterns
44e1e45 Update event_dispatcher.rst
f1b41ca Update event_dispatcher.rst
31a2315 Observer instead of Mediator pattern

Author: javiereguiluz

components/event_dispatcher.rst

@@ -25,9 +25,9 @@ before or after a method is executed, without interfering with other plugins.
 This is not an easy problem to solve with single inheritance, and even if
 multiple inheritance was possible with PHP, it comes with its own drawbacks.
 
-The Symfony EventDispatcher component implements the `Mediator`_ pattern
-in a simple and effective way to make all these things possible and to make
-your projects truly extensible.
+The Symfony EventDispatcher component implements the `Mediator`_ and `Observer`_
+design patterns to make all these things possible and to make your projects
+truly extensible.
 
 Take a simple example from :doc:`the HttpKernel component </components/http_kernel>`.
 Once a ``Response`` object has been created, it may be useful to allow other
@@ -523,6 +523,7 @@ Learn More
 * :ref:`The kernel.event_subscriber tag <dic-tags-kernel-event-subscriber>`
 
 .. _Mediator: https://en.wikipedia.org/wiki/Mediator_pattern
+.. _Observer: https://en.wikipedia.org/wiki/Observer_pattern
 .. _Closures: https://php.net/manual/en/functions.anonymous.php
 .. _PHP callable: https://php.net/manual/en/language.pseudo-types.php#language.types.callback
 .. _Packagist: https://packagist.org/packages/symfony/event-dispatcher