schema: fix reference to doc for universal macros

[JulieLorin]

It’s a small change, but I came across it while reviewing the schema structure.

[bokelley]

Thanks for the fix! Since this is a documentation-only change that doesn't affect the protocol or published schemas, you'll need to add an empty changeset to pass CI.

You can create one by running:

npx changeset add --empty

This creates a file in .changeset/ that signals to our version automation that this change doesn't need a version bump. Just commit and push that file.

Alternatively, you can manually create a file like .changeset/fix-universal-macros-doc.md with this content:

---
---

Fix reference to universal macros documentation in preview-creative-request schema.

(Note the empty frontmatter with just --- lines means no version bump.)

Let me know if you have any questions!

[JulieLorin]

@bokelley Thanks, it's done!