there is a large thread about this somewhere. I remember the Toyota team or someone from Toyota said 87 is fine and it was a misprint.
Here's the link with the entire discussion! The misprint is some where past the middle of the thread but don't hold me to that because it's been a long time! http://priuschat.com/forums/gen-iii-2010-prius-main-forum/61874-88-octane-2010-prius-whats-up.html