fix(dart): beforeSend code snippet (#11197)

Current code snippet example doesn't compile in dart

Author: buenaflor

platform-includes/user-feedback/sdk-api-example/dart.mdx

@@ -5,7 +5,7 @@ import 'package:sentry/sentry.dart';
 SentryId sentryId = SentryId.empty();
 
 await Sentry.init((options) {
-  options.beforeSend = (event, {hint}) async {
+  options.beforeSend = (event, hint) async {
     sentryId = event.eventId;
     return event;
   };