fix #7228 (typo): docs in validators.md to correct validate_default kwarg

[lmmx]

Docs typo referencing a nonexistent Field kwarg which should be validate_default.

Change Summary

Trivial typo fix, confirmed only instance in the repo.

Related issue number

• docs(typo): Field(validate_defaults=True) should be validate_default=True #7228

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review