cmd/gc: fix comment about how GOEXPERIMENT works
cmd/gc: fix comment about how GOEXPERIMENT works

LGTM=minux.ma
R=golang-codereviews, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/76270043

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