WAI-ARIA: Improve paragraph about aria-labelledby

[softy-dev]

Because

I found this paragraph could use a bit of improvement in how it's written to be more concise and clear.

This PR

• Modifies the initial paragraph about aria-labelledby.

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

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• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
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