Update copyright year

Author: mmistakes

README.md

@@ -954,7 +954,7 @@ grammar, etc.
 
 The MIT License (MIT)
 
-Copyright (c) 2017-2018 Michael Rose and contributors
+Copyright (c) 2017-2021 Michael Rose and contributors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal