Update Doc Copyright

Author: garyrussell

src/reference/asciidoc/index.adoc

@@ -14,7 +14,7 @@ ifdef::backend-html5[]
 *{project-version}*
 endif::[]
 
-(C) 2010 - 2020 by VMware, Inc.
+(C) 2010 - 2021 by VMware, Inc.
 
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