Update copyright statements for 2020
diff --git a/README.customs b/README.customs
index 4fd5fbb..a62e003 100644
--- a/README.customs
+++ b/README.customs
@@ -96,7 +96,7 @@
 
 
 -------------------------------------------------------------------------------
-Copyright (C) 1998-2019 Free Software Foundation, Inc.
+Copyright (C) 1998-2020 Free Software Foundation, Inc.
 This file is part of GNU Make.
 
 GNU Make is free software; you can redistribute it and/or modify it under the