JavaScript Asynchronous Code: clarify text for callbacks section link

[mozartdd]

Because

Clarifies the text for the callbacks section link — the previous version did not make it clear how much of the linked article should be read.

This PR

• Clarified the description for the callbacks section link in JavaScript/asynchronous_code.md.

Issue

Closes #29124

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• ] 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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[KevinMulhern]

Thanks @mozartdd