Fix typo

[nanashi-1]

Description

Fix typo systemd-tempfiles --create --prefix=/nix/var/nix

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

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[grahamc]

Oops, thank you for your PR! :)

[cole-h]

(Our CI is still busted for external contributors, so I cherry-picked your commit into #922, which will close this PR when merged -- sorry for the inconvenience, and thanks the fix!)