Replaced copyright notice

Author: pbuehler

PWGUD/Tasks/diffMCHelpers.h

@@ -1,3 +1,8 @@
+// Copyright 2019-2020 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
 // License v3 (GPL Version 3), copied verbatim in the file "COPYING".
 //
 // In applying this license CERN does not waive the privileges and immunities