doc/install: fix FreeBSD/Linux install command

R=adg, golang-dev, go.peter.90, rsc
CC=golang-dev
https://golang.org/cl/5700062

doc/install.html

@@ -75,7 +75,7 @@ Extract the archive into <code>/usr/local</code>, creating a Go tree in
 </p>
 
 <pre>
-tar -C /usr/local go.release.go1.tar.gz
+tar -C /usr/local -xzf go.release.go1.tar.gz
 </pre>
 
 <p>