enh(flutter): improve verify code snippet in getting started (#12369)

* Update index.mdx

* update dart snippet

Author: buenaflor

docs/platforms/flutter/index.mdx

@@ -123,7 +123,7 @@ Verify that your app is sending events to Sentry by adding the following snippet
 import 'package:sentry/sentry.dart';
 
 try {
-  aMethodThatMightFail();
+  throw Exception('Sentry Test Exception');
 } catch (exception, stackTrace) {
   await Sentry.captureException(
     exception,

platform-includes/capture-error/dart.mdx

@@ -4,7 +4,7 @@ In Dart you can capture any exception object that you caught:
 import 'package:sentry/sentry.dart';
 
 try {
-  aMethodThatMightFail();
+  throw Exception('Sentry Test Exception');
 } catch (exception, stackTrace) {
   await Sentry.captureException(
     exception,