Removed observer pattern, in favour of mediator

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | *
| Fixed tickets | 

I don't think this is the best tool to implement the observer pattern.

I just have copied the sentence from:
https://github.com/symfony/symfony-docs/edit/2.3/components/event_dispatcher/introduction.rst

but maybe could be improved with something like:

"Symfony Event Dispatcher is a tool that allows to make your projects truly extensible, facilitating the implementation of Mediator pattern and Publish subscribe pattern"

[edit-1]
I think this could be modified also here:
https://github.com/symfony/symfony-marketing/edit/master/views/en/get_started/components.html.twig