Fix a typo in https://github.com/flutter/engine/pull/27311

[moko256]

This PR fixes a typo from #27311.
#27311 (comment)

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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[flutter-dashboard]

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[Hixie]

test-exempt: code refactor with no semantic change