Bump copyright years.

zsh-syntax-highlighting.zsh

@@ -1,5 +1,5 @@
 # -------------------------------------------------------------------------------------------------
-# Copyright (c) 2010-2016 zsh-syntax-highlighting contributors
+# Copyright (c) 2010-2020 zsh-syntax-highlighting contributors
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without modification, are permitted