doc/contrib.html: fix some tags

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/14516057

doc/contrib.html

@@ -32,18 +32,18 @@ We encourage all Go users to subscribe to
 <h3 id="release"><a href="/doc/devel/release.html">Release History</a></h3>
 <p>A summary of the changes between Go releases.</p>
 
-<h4 id="go1notes"><a href="/doc/go1">Go 1 Release Notes</a></h3>
+<h4 id="go1notes"><a href="/doc/go1">Go 1 Release Notes</a></h4>
 <p>
 A guide for updating your code to work with Go 1.
 </p>
 
-<h4 id="go1.1notes"><a href="/doc/go1.1">Go 1.1 Release Notes</a></h3>
+<h4 id="go1.1notes"><a href="/doc/go1.1">Go 1.1 Release Notes</a></h4>
 <p>
 A list of significant changes in Go 1.1, with instructions for updating your
 code where necessary.
 </p>
 
-<h4 id="go1.2notes"><a href="/doc/go1.2">Go 1.2 Release Notes</a></h3>
+<h4 id="go1.2notes"><a href="/doc/go1.2">Go 1.2 Release Notes</a></h4>
 <p>
 A list of significant changes in Go 1.2, with instructions for updating your
 code where necessary.