chore(app-check): fix `firebase_app_check` iOS example app

[russellwheatley]

Description

See title.

Related Issues

See this issue for further details on setting up iOS app for debugging purposes. https://github.com/firebase/flutterfire/issues/8969

Checklist

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• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• [ ] All existing and new tests are passing.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] The analyzer (melos run analyze) does not report any problems on my PR.
• [ ] I read and followed the Flutter Style Guide.
• [ ] I signed the CLA.
• [ ] I am willing to follow-up on review comments in a timely manner.

Breaking Change

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• [ ] Yes, this is a breaking change.
• [X] No, this is not a breaking change.