Fix dart code snippet format (#7043)

`Sentry.init` accepts a callback function as an argument, so everything inside `{}` is a function flow, not an object. So every line should end with `;` and not `,`.

Author: MrLightful

src/platform-includes/performance/configure-sample-rate/dart.mdx

@@ -2,12 +2,12 @@
 import 'package:sentry/sentry.dart';
 
 Sentry.init((options) => {
-      options.dsn = '___PUBLIC_DSN___',
+      options.dsn = '___PUBLIC_DSN___';
       // To set a uniform sample rate
-      options.tracesSampleRate = 1.0,
+      options.tracesSampleRate = 1.0;
       // OR if you prefer, determine traces sample rate based on the sampling context
       options.tracesSampler = (samplingContext) {
         // return a number between 0 and 1 or null (to fallback to configured value)
-      },
+      };
     });
 ```