Fix median filter in cdeftutorial

Fixes commit 1198d31
What was described as an median filter was actually an average filter.

Sort the 3 values then take the middle of them using POP,EXC,POP.
Also uses one less prev() to decrease filter delay.

See https://en.wikipedia.org/w/index.php?title=Median_filter&oldid=558817813

Alternative fix is to rename "median" with "average" in the
description, but there already is an AVG rpn function for that.