Update copyright to 2021 in the about dialog

packages/help-extension/src/index.tsx

@@ -341,7 +341,7 @@ function activate(
       );
       const copyright = (
         <span className="jp-About-copyright">
-          {trans.__('© 2015-2020 Project Jupyter Contributors')}
+          {trans.__('© 2015-2021 Project Jupyter Contributors')}
         </span>
       );
       const body = (