fix(curriculum): correct async lecture text

[GandotraDiya]

Description

This PR fixes several factual, grammar, and wording issues in the "Understanding Asynchronous Programming" lecture block.

Changes include:

• Corrected inaccurate threading description in the asynchronous JavaScript lecture
• Improved wording and grammar in the Fetch API lecture
• Fixed incorrect idioms and awkward phrasing in the defer/async lecture
• Removed a grammatically broken clause in the promises lecture
• Fixed a missing article in the async/await lecture
• Improved wording in the JavaScript engine lecture
• Corrected redundant phrasing in the Geolocation API lecture

These updates improve technical accuracy, readability, and consistency across the curriculum content.

Testing

• I tested these changes locally
• These changes are text-only and do not affect functionality

Checklist:

• I have read and followed the contribution guidelines.
• I have read and followed the how to open a pull request guide.
• My pull request targets the main branch of freeCodeCamp.
• I have tested these changes either locally on my machine, or GitHub Codespaces.

Closes #67491

[github-actions]

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Closes #XXXXX

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[majestic-owl448]

LGTM