+ add a few missing words
+ pick the lowercase spelling for `stdenv` and use it consistenly
+ reword a sentence that I had a hard time understanding
+ fix the path of the second nix expression to be included

+ refer to the old nixpkgs commit in a note but use the current nixpkgs for the
  actual examples
+ add which to the PATH
+ add `unpackPhase = "true";` to satisfy stdenv requirements
+ explain how _PATH and PATH are related