Update dart.mdx

Author: buenaflor

platform-includes/performance/traces-sampler-as-filter/dart.mdx

@@ -1,7 +1,7 @@
 ```dart
 import 'package:sentry/sentry.dart';
 
-Sentry.init((options) {
+await Sentry.init((options) {
   // Determine traces sample rate based on the sampling context
   options.tracesSampler = (samplingContext) {
     final ctx = samplingContext.customSamplingContext;