gc: fix spelling of "GOEXPERIMENT" in a comment.
gc: fix spelling of "GOEXPERIMENT" in a comment.

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/5323070

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang