Update flex utilities link in navs docs

[kyletsang]

Description

Updates the flexbox utilities link in the horizontal alignment section to link directly to utilities/flex#justify-content. The previous link leads to the start of the grid page which looks incorrect.

Motivation & Context

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-38726--twbs-bootstrap.netlify.app/docs/5.3/components/navs-tabs/#horizontal-alignment

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