Update roundcube example configuration: smtp_server is deprecated

Related issue on GH: https://github.com/roundcube/roundcubemail/issues/8756

docs/add-roundcube.rst

@@ -20,7 +20,7 @@ servers may require more work.
         extraConfig = ''
           # starttls needed for authentication, so the fqdn required to match
           # the certificate
-          $config['smtp_server'] = "tls://${config.mailserver.fqdn}";
+          $config['smtp_host'] = "tls://${config.mailserver.fqdn}";
           $config['smtp_user'] = "%u";
           $config['smtp_pass'] = "%p";
         '';