README.md: update gethostname api
README.md: update gethostname api

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