Add missing import and fix typos in docs (#543)

docs/user-guide/stack-projects.md

@@ -44,7 +44,7 @@ instantiate a package set.
 let
   # Import the Haskell.nix library,
   haskellNix = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
-  nixpkgs = haskellNix.sources.nixpgs-1909 haskellNix.nixpkgsArgs;
+  nixpkgs = import haskellNix.sources.nixpkgs-1909 haskellNix.nixpkgsArgs;
   haskell = nixpkgs.haskell-nix;
 
   # Instantiate a package set using the generated file.