minor #8224 SiteUpdateManager should return $happyMessage and not the mailer service (revollat)

This PR was merged into the 3.3 branch.

Discussion
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SiteUpdateManager should return $happyMessage and not the mailer service

Commits
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e42435c SiteUpdateManager should return $happyMessage and not the mailer service

Author: xabbuh

service_container.rst

@@ -367,7 +367,7 @@ made. To do that, you create a new class::
                 );
             $this->mailer->send($message);
 
-            return $message;
+            return $happyMessage;
         }
     }