Fix broken install command for POSIX systems

[ndrew222]

Update installation instructions for downloading sn-cli.

[codacy-production]

Up to standards ✅

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[jonhadfield]

Thanks for catching this, the old command pointed at a file that didn't exist. It's since been fixed on main (bb82d09), and the README now has a one-line install.sh that picks the right archive and verifies its checksum, plus a Homebrew cask. As this now conflicts and the new instructions cover it, I'll close this, but thanks again for the report.