minor #8308 Update debugging.rst (atailouloute)

This PR was merged into the 2.7 branch.

Discussion
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Update debugging.rst

I noticed that the command `debug:event-dispatcher`is not present in the documentation page http://symfony.com/doc/current/debug/debugging.html#useful-debugging-commands

Commits
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69ce2a2 Update debugging.rst

Author: xabbuh

debug/debugging.rst

@@ -83,6 +83,9 @@ that can help you visualize and find the information.
     fallback translation for all known messages, if translations exist for
     the given locale.
 
+``debug:event-dispatcher``
+    Displays information about all the registered listeners in the event dispatcher.
+
 .. tip::
 
     When in doubt how to use a console command, open the help section by