style: Apply nixpkgs-fmt and fix

Apply standard formatting and some statix conventions using;

```sh
nixpkgs-fmt **.nix && statix fix .
```

With the intent of making contribution a bit easier and reducing mental
load in hand formatting (in the same vein as [black]).

[black]: https://github.com/psf/black#the-uncompromising-code-formatter

doc.nix

@@ -6,12 +6,12 @@ let
     rootMountPoint = "/mnt";
   };
   eval = lib.evalModules {
-    modules =  [
+    modules = [
       {
         options.disko = {
           devices = lib.mkOption {
             type = types.devices;
-            default = {};
+            default = { };
             description = "The devices to set up";
           };
         };
@@ -19,7 +19,7 @@ let
     ];
   };
   options = nixosOptionsDoc {
-    options = eval.options;
+    inherit (eval) options;
   };
   md = (runCommand "disko-options.md" { } ''
     cat >$out <<EOF
@@ -28,14 +28,15 @@ let
     EOF
     cat ${options.optionsCommonMark} >>$out
   '').overrideAttrs (o: {
-     # Work around https://github.com/hercules-ci/hercules-ci-agent/issues/168
-     allowSubstitutes = true;
+    # Work around https://github.com/hercules-ci/hercules-ci-agent/issues/168
+    allowSubstitutes = true;
   });
   css = fetchurl {
     url = "https://gist.githubusercontent.com/killercup/5917178/raw/40840de5352083adb2693dc742e9f75dbb18650f/pandoc.css";
     sha256 = "sha256-SzSvxBIrylxBF6B/mOImLlZ+GvCfpWNLzGFViLyOeTk=";
   };
-in runCommand "disko.html" { nativeBuildInputs = [ pandoc ]; } ''
+in
+runCommand "disko.html" { nativeBuildInputs = [ pandoc ]; } ''
   mkdir $out
   cp ${css} $out/pandoc.css
   pandoc --css="pandoc.css" ${md} --to=html5 -s -f markdown+smart --metadata pagetitle="Disko options" -o $out/index.html