doc: 2018 is the Year of the Gopher

Change-Id: Ie167512e000f3a8be0ff8a6a9edd52bd74f45115
Reviewed-on: https://go-review.googlesource.com/85775Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

doc/contribute.html

@@ -394,7 +394,7 @@ Authors”—the copyright holders—are.
 <p>New files that you contribute should use the standard copyright header:</p>
 
 <pre>
-// Copyright 2017 The Go Authors. All rights reserved.
+// Copyright 2018 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>