fix alert-color in v5.3.0-alpha2

[cccabinet]

Description

Change the alert text color from $themecolor-text to $themecolor-text-emphasis.

Motivation & Context

Alert text color is default text color.
Perhaps you forgot to change this when you changed $theme-colors-text from $themecolor-text to $themecolor-text-emphasis in #37907.

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-38003--twbs-bootstrap.netlify.app/docs/5.3/components/alerts/

Related issues

[mdo]

Thanks!