docs: update archlinux commands

[george124816]

Description

Since have a trivy on archlinux official communtiy repository, why not recommend to install using pacman?

docker run -it --rm archlinux bash -c "pacman -Sy && pacman -Sy --noconfirm trivy"

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Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

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[knqyf263]

Thanks!