doc: update standard copyright header

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/7034044

doc/contribute.html

@@ -530,7 +530,7 @@ This rigmarole needs to be done only for your first submission.
 <p>Code that you contribute should use the standard copyright header:</p>
 
 <pre>
-// Copyright 2012 The Go Authors. All rights reserved.
+// Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 </pre>