Fix comment in Parallel Letter Frequency Tests

[rzuckerm]

See https://forum.exercism.org/t/missing-in-parallel-letter-frequence-test/19853

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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[no important files changed]

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[github-actions]

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[github-actions]

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