docs: add domEnvironment option to example

[tillsanders]

🔗 Linked issue

No issue, just a minor addition to the documentation.

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The Testing Guide mentions that it is possible to choose between happy-dom and jsdom, but it did not mention how. Since I was using jsdom before and happy-dom was not installed, all my tests failed. I had to go through the source code to discover the domEnvironment option to fix this.

Maybe I'm missing something, but it feels like the Testing Guide could in general use a bit more detail here and there.

📝 Checklist

• I have linked an issue or discussion.
• I have added tests (if possible).
• I have updated the documentation accordingly.

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