flutter: fix dsn placeholder (#14040)

Should be `___DSN___`

Author: buenaflor

docs/platforms/dart/guides/flutter/session-replay/index.mdx

@@ -33,7 +33,7 @@ To set up replay, enable it by setting a non-zero sample rate and wrapping your
 ```dart
 await SentryFlutter.init(
   (options) {
-    options.dsn = '__DSN__';
+    options.dsn = '___DSN___';
     options.replay.sessionSampleRate = 1.0;
     options.replay.onErrorSampleRate = 1.0;
   },