Docs: fix specification for getInstance

[fulldecent]

Description

Fixes documentation for Tooltip in docs.

getInstance had wrong documentation.

Motivation & Context

I confirmed the actual implementation matches my new documentation:

https://github.com/twbs/bootstrap/blob/53171ad564b2d01bff7613d7210e93a5197a367b/js/src/base-component.js#L59-L66

Type of changes

• [x] 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

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

Live previews

• https://deploy-preview-41297--twbs-bootstrap.netlify.app/

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