doc: change "threads" to "goroutines"
doc: change "threads" to "goroutines"

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/40510049

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang