add use statement

Author: erlangparasu

components/mailer.rst

@@ -60,6 +60,7 @@ it:
 
 Then, use the SMTP Gmail transport::
 
+    use Symfony\Component\Mailer\Mailer;
     use Symfony\Component\Mailer\Bridge\Google\Smtp\GmailTransport;
 
     $transport = new GmailTransport('user', 'pass');