Update copyright, use en dash for date range.

Author: RKrahl

doc/src/about.rst

@@ -69,9 +69,9 @@ those results that really matter.
 Copyright and License
 ---------------------
 
-- Copyright 2013-2015
+- Copyright 2013–2015
   Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
-- Copyright 2016-2018 Rolf Krahl
+- Copyright 2016–2020 Rolf Krahl
 
 Licensed under the Apache License, Version 2.0 (the "License"); you
 may not use this file except in compliance with the License.  You may