minor symfony#21096 Fix php example syntax error at mailer.rst (diimpp)

This PR was merged into the 6.4 branch.

Discussion
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Fix php example syntax error at `mailer.rst`

Fixes syntax error at php example at `mailer.rst`

Commits
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da10e09 Fix php example syntax error at mailer.rst

Author: OskarStark

mailer.rst

@@ -1765,7 +1765,7 @@ the HTTP calls made by the HTTP transports, which is useful for debugging errors
 
     public function onMessage(SentMessageEvent $event): void
     {
-        $message $event->getMessage();
+        $message = $event->getMessage();
 
         // do something with the message (e.g. get its id)
     }