Update the NOTICE file to 2021 for copyright.

Very simple fix to update the year in the NOTICE file to 2021.

Fixes #2624 

Reviewers: Enrico Olivelli <eolivelli@gmail.com>

This closes #2621 from hsaputra/update_notice_year_2021
diff --git a/NOTICE b/NOTICE
index c2788b6..dd8d6e3 100644
--- a/NOTICE
+++ b/NOTICE
@@ -1,5 +1,5 @@
 Apache BookKeeper
-Copyright 2011-2020 The Apache Software Foundation
+Copyright 2011-2021 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).