doc: replace hg commands with git commands in release.html

Change-Id: I5cc4740a858c40d820eeb7c00cf0508b20807c90
Reviewed-on: https://go-review.googlesource.com/1533Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/devel/release.html

@@ -8,8 +8,8 @@ The <a href="//golang.org/change">change log</a> has the full details.</p>
 <p>To update to a specific release, use:</p>
 
 <pre>
-hg pull
-hg update <i>tag</i>
+git pull
+git checkout <i>release-branch</i>
 </pre>
 
 <h2 id="go1.4">go1.4 (released 2014/12/10)</h2>