使用了How do I change the CDN used in my flarum?中Toby提到的方法，启动extension后没有效果，是插件的写法有问题吗？
Bootstrap.php
<?php
use Illuminate\Contracts\Events\Dispatcher;
return function (Dispatcher $events) {
$events->listen(ConfigureClientView::class, function (ConfigureClientView $event) {
$event->view->addHeadString('<link rel="stylesheet" href="//fonts.lug.ustc.edu.cn/css?family=Open+Sans:400italic,700italic,400,700,600">', 'font');
});
};