rcverify: update expected end year in NOTICE.txt (#367)

diff --git a/tools/rcverify.sh b/tools/rcverify.sh
index 1037d7f..3a1851f 100755
--- a/tools/rcverify.sh
+++ b/tools/rcverify.sh
@@ -69,7 +69,7 @@
 
 NOTICE=$(cat << END
 Apache $DESCRIPTION
-Copyright 2016-2020 The Apache Software Foundation
+Copyright 2016-2021 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).