Fix undefined offset

Items in $warnings should either be a string or an array with two values

modules/core/www/frontpage_config.php

@@ -61,7 +61,7 @@ $allLinks = [
 $current = $config->getVersion();
 if ($config->getBoolean('admin.checkforupdates', true) && $current !== 'master') {
     if (!function_exists('curl_init')) {
-        $warnings[] = ['{core:frontpage:warnings_curlmissing}'];
+        $warnings[] = '{core:frontpage:warnings_curlmissing}';
     } else {
         $latest = $session->getData("core:latest_simplesamlphp_version", "version");