docs: fix grammar error in CONTRIBUTING.md

[WuMingDao]

🎯 Changes

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

[changeset-bot]

⚠️ No Changeset found

Latest commit: 24aece9

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[LeCarbonator]

Thank you!

[autofix-ci]

Hi! I'm autofix.ci, a bot that automatically fixes trivial issues such as code formatting in pull requests.

I would like to apply some automated changes to this pull request, but it looks like I don't have the necessary permissions to do so. To get this pull request into a mergeable state, please do one of the following two things:

1. Allow edits by maintainers for your pull request, and then re-trigger CI (for example by pushing a new commit).
2. Manually fix the issues identified for your pull request (see the GitHub Actions output for details on what I would like to change).