docs: update zpl-2-0 link

Author: rouilj

COPYING.txt

@@ -49,7 +49,7 @@ The license for this code is the `Zope Public License (ZPL) Version 2.0`_,
 included below.
 
 
-.. _Zope Public License (ZPL) Version 2.0: https://opensource.org/license/zpl-2-0/
+.. _Zope Public License (ZPL) Version 2.0: https://opensource.org/license/zpl-2-0
 
 Zope Public License (ZPL) Version 2.0
 -------------------------------------