doc/install: fix FreeBSD/Linux install command
doc/install: fix FreeBSD/Linux install command

R=adg, golang-dev, go.peter.90, rsc
CC=golang-dev
http://codereview.appspot.com/5700062

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go