docs: Add quotes to the installation command

[duzhuoshanwai]

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pip install 'httpx[socks]'

Summary

Checklist

• [✅] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [✅] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [✅] I've updated the documentation accordingly.