Docs: Update test-runner.md prepare function typo

[kota-kamikawa]

One parenthesis was unnecessary.

Closes #

What I did

I have fixed prepare function typo in test-runner.md.

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  MIGRATION.MD

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[jonniebigodes]

@kota-kamikawa thanks for taking the time to put together this pull request and helping us improve the documentation by catching this small typo. We appreciate it. I've checked and all is good on my end. I'll gladly merge this once the checklist clears.

Hope you have a fantastic day.

Stay safe