Fix stack examples

[kyletsang]

Description

I was going over the stack docs and noticed that the first two horizontal stack examples were the same. I think the .ms-auto class was accidentally removed in https://github.com/twbs/bootstrap/pull/37900/files

Motivation & Context

Fixes examples so it matches wording.

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-38745--twbs-bootstrap.netlify.app/docs/5.3/helpers/stacks/#horizontal

Related issues

[julien-deramond]

Good catch! Thanks for the fix @kyletsang