updated website footer to read 2021 (#4959)

Author: Aaronontheweb

docs/docfx.json

@@ -68,7 +68,7 @@
             "_appTitle": "Akka.NET Documentation",
             "_appLogoPath": "/images/akkalogo.png",
             "_appFaviconPath": "/images/favicon.ico",
-            "_appFooter": "Copyright © 2013-2019 Akka.NET project<br>Generated by <strong>DocFX</strong>",
+            "_appFooter": "Copyright © 2013-2021 Akka.NET project<br>Generated by <strong>DocFX</strong>",
             "_enableSearch": "true"
         },
         "dest": "_site",