minor #21508 [EventDispatcher][Event Listeners] Update event_dispatcher.rst : fix return type (Glioburd)

This PR was submitted for the 7.3 branch but it was merged into the 6.4 branch instead.

Discussion
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[EventDispatcher][Event Listeners] Update event_dispatcher.rst : fix return type

Fixed return type for a setter in the example `BeforeSendMailEvent` class. From `string` to `void`.

Commits
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e834cf0 Update event_dispatcher.rst : fix return type

Author: javiereguiluz

event_dispatcher.rst

@@ -728,7 +728,7 @@ this::
             return $this->subject;
         }
 
-        public function setSubject(string $subject): string
+        public function setSubject(string $subject): void
         {
             $this->subject = $subject;
         }