Update "offline" message

Summary:
Offline doesn't really mean anything in this context. The problem isn't that you're offline, it's that the server isn't reachable.

Changelog: Update offline message

Reviewed By: antonk52

Differential Revision: D59629955

fbshipit-source-id: d399470e506d1359ff873f7528b88a1a641aa9e6

desktop/static/offline.html

@@ -6,7 +6,7 @@
   <meta http-equiv="X-UA-Compatible" content="IE=edge" />
   <meta name="viewport" content="width=device-width, initial-scale=1" />
 
-  <title>You are offline</title>
+  <title>Flipper Server not running</title>
 
   <!-- Inline the page's stylesheet. -->
   <style>