doc/install.html: fix link

Fixes #3072.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5684044

doc/install.html

@@ -144,7 +144,7 @@ If you see the "hello, world" message then your Go installation is working.
 
 <p>
 Start by taking <a href="http://code.google.com/p/go-tour/">A Tour of Go</a>
-or reading the <a href="go_tutorial.html">Go Tutorial</a>.
+or reading the <a href="/doc/go_tutorial.html">Go Tutorial</a>.
 </p>
 
 <p>