Fix #945 - documentation fix
Fix #945 - documentation fix

Correct references to user ids to group ids.

geofft/nix-rust,asomers/nix,xd009642/nix,carllerche/nix-rust,nix-rust/nix,nix-rust/nix,asomers/nix,xd009642/nix,geofft/nix-rust,geofft/nix-rust,xd009642/nix,MarkusJais/nix-rust,carllerche/nix-rust,carllerche/nix-rust,MarkusJais/nix-rust,MarkusJais/nix-rust