minor symfony#12706 add use statement (erlangp)

This PR was submitted for the 5.0 branch but it was squashed and merged into the 4.3 branch instead (closes symfony#12706).

Discussion
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add use statement

Commits
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a2ea8a7 add use statement

Author: OskarStark

components/mailer.rst

@@ -66,6 +66,7 @@ it:
 Then, use the SMTP Gmail transport::
 
     use Symfony\Component\Mailer\Bridge\Google\Smtp\GmailTransport;
+    use Symfony\Component\Mailer\Mailer;
 
     $transport = new GmailTransport('user', 'pass');
     $mailer = new Mailer($transport);