That is for when users are subscribed to a forum, not a particular thread. The post text is not grabbed in the query for the forum subscriptions and there are no hooks available. That means you will have to modify the actual query to grab the post text and then modify the code to parse it for the email and then modify the phrase to spit it out. The query is around line 204 in includes/functions_digest.php. I think you'll need to JOIN the post table in order to get the post text of the first post (I assume that is all you are wanting to grab).