doc: fix the closing tag in contribute.html

Change-Id: I802bc11d90d414e7313dd11842a04fd6b591ea06
Reviewed-on: https://go-review.googlesource.com/90595Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -30,7 +30,7 @@ You must go through the following process <em>prior to contributing</em>.
 You only need to do this once per Google Account.
 </p>
 
-<h2 id="go-contrib-init">Automatically set up &amp; diagnose your development environment</h3>
+<h2 id="go-contrib-init">Automatically set up &amp; diagnose your development environment</h2>
 <p>
   The <code>go-contrib-init</code> tool configures and debugs your Go
   development environment, automatically performing many of the steps