Change sync_code_signing example to use an array type

[mpirri]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Because the sync_code_signing exports the support ConfigItem types from the match tool, app_identifier is expected to be an Array. To prevent docs build from failing, change the sample code to use an array.

[fastlane-bot]

Hey @mpirri 👋

Thank you for your contribution to fastlane and congrats on getting this pull request merged 🎉
The code change now lives in the master branch, however it wasn't released to RubyGems yet.
We usually ship about once a week, and your PR will be included in the next one.

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[fastlane-bot]

Congratulations! 🎉 This was released as part of fastlane 2.69.1 🚀