ServerPushSetupBanner: Fix conditional for showing admin-facing message

Somehow I got this conditional backwards when I wrote it in
cb301ec.

Author: chrisbobbe

src/common/ServerPushSetupBanner.js

@@ -50,11 +50,11 @@ export default function ServerPushSetupBanner(props: Props): Node {
     visible = true;
     text = isAtLeastAdmin
       ? {
-          text: 'The Zulip server at {realm} is not set up to deliver push notifications. Please contact your administrator.',
+          text: 'The Zulip server at {realm} is not set up to deliver push notifications.',
           values: { realm: realm.toString() },
         }
       : {
-          text: 'The Zulip server at {realm} is not set up to deliver push notifications.',
+          text: 'The Zulip server at {realm} is not set up to deliver push notifications. Please contact your administrator.',
           values: { realm: realm.toString() },
         };
   }