xerrors: update dates in LICENSE

Change-Id: Ie73aa07954d378de165855fa5cd8313f36e2369f
Reviewed-on: https://go-review.googlesource.com/c/160217
Reviewed-by: Ian Lance Taylor <iant@golang.org>

LICENSE

diff --git a/LICENSE b/LICENSE
index 6a66aea..e4a47e1 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,4 +1,4 @@
-Copyright (c) 2009 The Go Authors. All rights reserved.
+Copyright (c) 2019 The Go Authors. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are