Fix the set-user scope example in Dart

[timekone]

This is how the set-user scope example looks right now:

import 'package:sentry/sentry.dart';

Sentry.configureScope(
  (scope) => scope.user = User(id: 1234, email: 'jane.doe@example.com'),
);

But User.id is a String, so there are missing quotation marks around 1234.
How it should look:

import 'package:sentry/sentry.dart';

Sentry.configureScope(
  (scope) => scope.user = User(id: "1234", email: 'jane.doe@example.com'),
);

#3244 may be related.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/sentry/sentry-docs/5NjQyoHpQYTsVrj1t84YSxnVUYn5
✅ Preview: https://sentry-docs-git-fork-timekone-timekone-patch-1.sentry.dev

[marandaneto]

@timekone good catch, also fixed on #3244 thanks for that