doc: 2019 is the Year of the Gopher

whereas this is a longstanding tradition
and insofaras it is worth continuing such traditions
and notwithstanding an attempt at future-proofing
thetruthofthematter is that I have been waiting for years to send this change
so despiteallobjections I have updated the copyright year.

Change-Id: I55961b15a7eda35d84fdd9250afdbe19f0bf8412
Reviewed-on: https://go-review.googlesource.com/c/155928Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

doc/contribute.html

@@ -922,13 +922,13 @@ New files that you contribute should use the standard copyright header:
 </p>
 
 <pre>
-// Copyright 2018 The Go Authors. All rights reserved.
+// Copyright 2019 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 2019 or beyond.)
+(Use the current year if you're reading this in 2020 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>