doc/install: specify clone -u instead of -r

R=rsc
CC=golang-dev
https://golang.org/cl/4435081

doc/install.html

@@ -136,7 +136,7 @@ and make sure the <code>go</code> directory does not exist.
 Then check out the repository:</p>
 
 <pre>
-$ hg clone -r release https://go.googlecode.com/hg/ go
+$ hg clone -u release https://go.googlecode.com/hg/ go
 </pre>
 
 <h2 id="install">Install Go</h2>