login: Use "your-org.zulipchat.com" for realm-url hint, like zulip-mobile

Fixes-partly: zulip#35

Author: chrisbobbe

lib/widgets/login.dart

@@ -187,6 +187,7 @@ class _AddAccountPageState extends State<AddAccountPage> {
               keyboardType: TextInputType.url,
               decoration: InputDecoration(
                 labelText: 'Your Zulip server URL',
+                hintText: 'your-org.zulipchat.com',
                 errorText: error == null || error.shouldDeferFeedback()
                   ? null
                   : error.message(),