fix: fix non-flake install instructions

[es-sai-fi]

Forgot to pass an empty attrSet, I apologize.

[greptile-apps]

Greptile Summary

Fixed the non-flake NixOS installation instructions by adding an empty attribute set {} to the import sources.gitv statement. This corrects the Nix syntax to properly call the imported expression, which expects to receive an attribute set as an argument. The fix aligns with the pattern shown in the "No Pinning Tool" example and ensures the installation instructions will work correctly.

Important Files Changed

Filename	Overview
README.md	Fixed Nix import syntax by adding empty attribute set to sources.gitv import

_{Last reviewed commit: 1a6773d}

[greptile-apps]

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