update end year in NOTICE file to 2020 (#41)

diff --git a/NOTICE.txt b/NOTICE.txt
index 766e4cc..8183e4b 100644
--- a/NOTICE.txt
+++ b/NOTICE.txt
@@ -1,5 +1,5 @@
 Apache OpenWhisk Runtime Dotnet
-Copyright 2016-2019 The Apache Software Foundation
+Copyright 2016-2020 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).