minor #9337 Update service_container.rst (ilmoralito, javiereguiluz)

This PR was submitted for the 4.0 branch but it was merged into the 3.4 branch instead (closes #9337).

Discussion
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Update service_container.rst

Since 6 use `$message = (new \Swift_Message('message'))` instead of `$message = \Swift_Message::newInstance()`

Please check https://github.com/swiftmailer/swiftmailer/blob/master/CHANGES#L24 and https://swiftmailer.symfony.com/docs/introduction.html#basic-usage

Commits
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e4ba061 Fixed the code
17da22a Update service_container.rst

Author: javiereguiluz

service_container.rst

@@ -364,8 +364,7 @@ made. To do that, you create a new class::
         {
             $happyMessage = $this->messageGenerator->getHappyMessage();
 
-            $message = \Swift_Message::newInstance()
-                ->setSubject('Site update just happened!')
+            $message = (new \Swift_Message('Site update just happened!))
                 ->setFrom('admin@example.com')
                 ->setTo('manager@example.com')
                 ->addPart(