include: fix comment in link.h
include: fix comment in link.h

LGTM=dave, rsc
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/68960044

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