Fix license_header.txt file.

docs/license_header.txt

diff --git a/docs/license_header.txt b/docs/license_header.txt
index a0fce4a..5c5e95e 100644
--- a/docs/license_header.txt
+++ b/docs/license_header.txt
@@ -1,4 +1,4 @@
-Copyright 2021 SkyWater PDK Authors
+Copyright 2019-2021 SkyWater PDK Authors
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.