fix(flutter): update Flutter reportFullyDisplayed api (#11821)

Author: buenaflor

docs/platforms/flutter/integrations/routing-instrumentation.mdx

@@ -132,7 +132,7 @@ await SentryFlutter.init(
 2. Report the span manually:
 
 ```dart
-SentryFlutter.reportFullyDisplayed();
+await SentryFlutter.reportFullyDisplayed();
 ```
 
 If the span finishes through the API, its status will be set to `SpanStatus.OK`.