Mention pagination flag for classroom cli student repo downloads

[bertheyman]

Why:

The default pagination setting of 30 when downloading student repositories causes confusion for users.
I couldn't find this in the docs yet, so this PR guides users to use the --per-page flag.

Closes: github/gh-classroom#4

What's being changed (if available, include any code snippets, screenshots, or gifs):

An extra line in the docs informing about the --per-page flag when downloading student repositories.

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Source	Preview	Production	What Changed
education/manage-coursework-with-github-classroom/teach-with-github-classroom/using-github-classroom-with-github-cli.md	fpt	fpt

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ghec: GitHub Enterprise Cloud
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[cmwilson21]

@bertheyman Thanks so much for submitting a PR! I'll get this triaged for review ⚡

[sophietheking]

@bertheyman 👋 Thanks for opening this PR and contributing to our Docs, we really appreciate it!

This change looks good to me, so I'll get it merged. 🚀

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