minor symfony#11241 Update link to rack website (yesnik)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes symfony#11241).

Discussion
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Update link to rack website

Previous link to rubyforge.org is broken, because this project was closed at May 2014.
New link contains information about Rack, a modular Ruby webserver interface.

Commits
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73473b7 Update link to rack website

Author: javiereguiluz

create_framework/event_dispatcher.rst

@@ -301,4 +301,4 @@ is not about creating a generic framework, but one that is tailored to your
 needs. Stop whenever you see fit, and further evolve the code from there.
 
 .. _`WSGI`: https://www.python.org/dev/peps/pep-0333/#middleware-components-that-play-both-sides
-.. _`Rack`: http://rack.rubyforge.org/
+.. _`Rack`: https://github.com/rack/rack