First of all - there is no post-credits scene in Avengers: Age of Ultron at all. None. There is, however, a very clever hoax being delivered to the web this week courtesy of one serious hoaxer. How exactly they created the sequence you're about to see, we do not know - but it's fairly believable. Especially since this is the quality at which most "post credits sequences" are delivered these days, generally speaking. And it wouldn't make much sense for Marvel to reboot Spider-Man like this AGAIN, would it?