matching issue tracker to top-level samples README (#2405)

*Replace this paragraph with a description of what this PR is changing
or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. For larger changes, raising an
issue first helps
reduce redundant work.*

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

add_to_app/README.md

@@ -54,4 +54,4 @@ functionality, similar to a full Flutter app.
 ## Issues
 
 If you run into an issue with the sample itself, please file an issue
-in the [main Flutter repo](https://github.com/flutter/flutter/issues).
+in the [Flutter samples repo](https://github.com/flutter/samples/issues).