[racket] 3D Plot example doesn't work in Typed Racket

From: Greg Hendershott (greghendershott at gmail.com)
Date: Fri May 30 23:25:27 EDT 2014

Thanks, Neil and Alexander!

(At one point I actually did try `list` instead of `vector`, but that
uncovered an additional typecheck problem that I conflated with this.
All set now.)

Posted on the users mailing list.