Fix docs of arch_package_manager fn

[Akeshihiro]

Standards checklist:

• [ ] The PR title is descriptive.
• [ ] The code compiles (cargo build)
• [ ] The code passes rustfmt (cargo fmt)
• [ ] The code passes clippy (cargo clippy)
• [ ] The code passes tests (cargo test)
• [ ] Optional: I have tested the code myself
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