Update copyright date in reference manual

Author: sbrannen

src/docs/asciidoc/index.adoc

@@ -32,7 +32,7 @@ Brannen, Ramnivas Laddad, Arjen Poutsma, Chris Beams, Tareq Abedrabbo, Andy Clem
 Syer, Oliver Gierke, Rossen Stoyanchev, Phillip Webb, Rob Winch, Brian Clozel, Stephane
 Nicoll, Sebastien Deleuze, Jay Bryant, Mark Paluch
 
-Copyright © 2002 - 2020 Pivotal, Inc. All Rights Reserved.
+Copyright © 2002 - 2021 Pivotal, Inc. All Rights Reserved.
 
 Copies of this document may be made for your own use and for distribution to others,
 provided that you do not charge any fee for such copies and further provided that each