Fixed the code

Author: javiereguiluz

service_container.rst

@@ -364,10 +364,12 @@ made. To do that, you create a new class::
         {
             $happyMessage = $this->messageGenerator->getHappyMessage();
 
-            $message = (new \Swift_Message('Someone just updated the site. We told them: '.$happyMessage))
-                ->setSubject('Site update just happened!')
+            $message = (new \Swift_Message('Site update just happened!))
                 ->setFrom('admin@example.com')
-                ->setTo('manager@example.com');
+                ->setTo('manager@example.com')
+                ->addPart(
+                    'Someone just updated the site. We told them: '.$happyMessage
+                );
 
             return $this->mailer->send($message) > 0;
         }