Fixes error message if no combine is passed to materializer

[skrawcz]

I had a typo and the error was cryptic. This fixes that to explain that the combine= kwarg needs to be used.

Changes

• materialization.py

How I tested this

• locally

Notes

• I don't like _build_result being the default node name post combine.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.