manual: "Nix Package collection" -> "Nixpkgs package collection".
manual: "Nix Package collection" -> "Nixpkgs package collection".

Makes difference between Nix and Nixpkgs clearer to avoid
some common confusion this sentence on IRC.

Also disambiguate an "it" reference.

vcunat/nix,ehmry/nix,vcunat/nix,ehmry/nix,ehmry/nix,NixOS/nix,rycee/nix,ehmry/nix,vcunat/nix,dtzWill/nix,NixOS/nix,NixOS/nix,pikajude/nix,NixOS/nix,ehmry/nix,rycee/nix,vcunat/nix,dtzWill/nix,pikajude/nix,NixOS/nix,rycee/nix,dtzWill/nix,ehmry/nix,rycee/nix,pikajude/nix,dtzWill/nix,dtzWill/nix,rycee/nix