Update flutter.mdx

Author: buenaflor

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

@@ -1,5 +1,5 @@
 ```dart
-SentryFlutter.init((options) {
+await SentryFlutter.init((options) {
   // Determine traces sample rate based on the sampling context
   options.tracesSampler = (samplingContext) {
     final ctx = samplingContext.customSamplingContext;