Docs: Fix incorrect class name on migration guide

[coliff]

Description

There wasn't a Bootstrap v4 class called .form-file - it was .form-control-file.

REF: https://getbootstrap.com/docs/4.6/dist/css/bootstrap.css

Motivation & Context

Help users upgrading by finding the right file in their code.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38883--twbs-bootstrap.netlify.app/

Related issues

[julien-deramond]

Thanks @coliff, good catch!