public function onContentAfterTitle($context, &$article, &$params, $limitstart)
{
return "<p>Hello World</p>";
}

If you want to insert codes before article content displayed then
user this event -

public function onContentBeforeDisplay($context, &$row, &$params, $limitstart)
{
return '<div>ad code1</div>';
}

But if you want codes to be appared after article content then this -

public function onContentAfterDisplay($context, &$row, &$params, $limitstart)
{
return "<div>ad code 2</div>";
}

Hope that helped you very much.If it helped you in greater extent
then share and like at least