docs(messaging): update readme

[supanadit]

Description

Anyone who just need firebase messaging then following the guide only and haven't experience with it, they will found an error which is at function myBackgroundMessageHandler, and they don't know what is the problem, i had got this problem twice at different project, so i don't want anyone who just follow the basic guide got an exact same error because of forgotten async word.

Related Issues

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Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• 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 (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
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