docs: Fix typo in __get_cpuid_max description

[tyilo]

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[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @Amanieu (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

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The reviewer was selected based on:

• Owners of files modified in this PR: @Amanieu, @adamgemmell, @davidtwco, @folkertdev, @sayantn
• @Amanieu, @adamgemmell, @davidtwco, @folkertdev, @sayantn expanded to Amanieu, adamgemmell, davidtwco, folkertdev, sayantn
• Random selection from Amanieu, adamgemmell, davidtwco, folkertdev, sayantn