add async modifier

Author: denrase

platform-includes/capture-error/flutter.mdx

@@ -13,12 +13,12 @@ If you need a custom error handling zone which also provides automatic error rep
 This function automatically records calls to `print()` as `Breadcrumbs` and can be configured using `SentryOptions.enablePrintBreadcrumbs`.
 
 ```dart
-Sentry.runZonedGuarded(() {
+Sentry.runZonedGuarded(() async {
   WidgetsBinding.ensureInitialized();
 
   // Errors before init will not be handled by Sentry
 
-  SentryFlutter.init(
+  await SentryFlutter.init(
     (options) {
     ...
     },