Fix populate_by_name migration note about old name

[schneebuzz]

Change Summary

Docs change only: Changed note about name of populate_by_name config field before pydantic v2.x.

Related issue number

No issue created for trivial doc change.

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 (no code changes)
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[schneebuzz]

please review

[adriangb]

Thanks!