fix: apply missing ) to DevtoolsPanel example code

[TakhyunKim]

🎯 Changes

Fixes: #186

The custom plugin documentation contains the following typo:

• jsx closed typo in DevtoolsPanel.ts example code
• on method ) missing in DevtoolsPanel.ts example code

✅ 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]

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