doc: 2020 is the Year of the Gopher

Starting with 2014 (golang.org/cl/46660043), we have enjoyed 6
consecutive years of the gopher. Now, the slice¹ of gophers is
ready to make its way into the next decade, as 2020 is the new
Year of the Gopher.

¹ https://en.wikipedia.org/w/index.php?title=List_of_English_terms_of_venery,_by_animal&oldid=932675028#G

Change-Id: I5f9598dbedb373bd13021964193fa9e44c67693e
Reviewed-on: https://go-review.googlesource.com/c/go/+/213017
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Symonds <dsymonds@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/contribute.html

@@ -925,13 +925,13 @@ New files that you contribute should use the standard copyright header:
 </p>
 
 <pre>
-// Copyright 2019 The Go Authors. All rights reserved.
+// Copyright 2020 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>
 
 <p>
-(Use the current year if you're reading this in 2020 or beyond.)
+(Use the current year if you're reading this in 2021 or beyond.)
 Files in the repository are copyrighted the year they are added.
 Do not update the copyright year on files that you change.
 </p>