Update quickstart.md

[e1codes]

Summary

Fixed #1477 "Generating Markdown Output" example code from quick start (https://docs.crawl4ai.com/core/quickstart/)

List of files changed and why

Python code example in link below changed. The problem was with incorrect use of async function(must be declared), so the code couldn't run.
https://github.com/unclecode/crawl4ai/blob/develop/docs/md_v2/core/quickstart.md#4-generating-markdown-output

How Has This Been Tested?

Ran the updated code multiple times, and it works properly.

Checklist:

• [x ] My code follows the style guidelines of this project
• [x ] I have performed a self-review of my own code
• [x ] I have made corresponding changes to the documentation

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✨ Finishing touches

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment

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[unclecode]

Closing as duplicate of #1715. Thanks for the contribution.