Update adding-a-fieldtype.md

[jonathoncove3]

Changed reference to preValues , based on experience in umb 13.8.1

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I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
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• Typos, broken links, and broken images are fixed.

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• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[sofietoft]

Thanks for the PR @jonathoncove3 !

We'll get this reviewed as soon as possible.

[sofietoft]

Thanks again for the PR @jonathoncove3 !

I'll get this merged 💪