add_filter instant_articles_content

I try this fuction in function.php but not appear my credit…

function alter_instant_articles_content($content) {
// magic happen here…
if (is_single()) {
$content .= ‘© KSN-NEWS.COM’;
}
return $content;
}
add_filter(‘instant_articles_content’, ‘alter_instant_articles_content’,10, 3);

https://ww.wp.xz.cn/plugins/fb-instant-articles/