Update add-dns.php for newer db schema

pg-act/ht/add-dns.php

@@ -5,7 +5,7 @@ $_POST['domain'] = formatDomain($_POST['domain']);
 if (dirsStatuses('dns')[$_POST['dir']] !== false)
 	output(403, 'Wrong value for <code>dir</code>.');
 
-if (query('select', 'sites', ['domain' => $_POST['domain']], 'domain') !== [])
+if (query('select', 'sites', ['address' => $_POST['domain']], 'address') !== [])
 	output(403, _('This domain already exists on this service. Use another one.'));
 
 $remoteAaaaRecords = dns_get_record($_POST['domain'], DNS_AAAA);