docs(eval): fix evaluator plugin docs so code compiles

[odbol]

Fix some errors and mismatched types so that if someone copy/pastes all the code from the doc, it will at least compile and run

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)