Fix pydantic-settings to underscore in docs

[FacerAin]

Change Summary

In this migration docs, it seems desirable to change "pydantic-settings" to "pydantic_settings" with underscores.
Even though the package repo name is "pydantic-settings", In Python, we still need to use underscores as shown below.

from pydantic_settings import BaseSettings

I believe this change will be helpful for users.

Thanks! ☺️

Related issue number

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, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[FacerAin]

please review

[hramezani]

Thanks @FacerAin