doc:fhs-envs: format example properly

That sort of formatting wouldn't be accepted into Nixpkgs. We shouldn't format
an example like that.

doc/builders/special/fhs-environments.section.md

@@ -29,18 +29,18 @@ One can create a simple environment using a `shell.nix` like that:
 
 (pkgs.buildFHSEnv {
   name = "simple-x11-env";
-  targetPkgs = pkgs: (with pkgs;
-    [ udev
-      alsa-lib
-    ]) ++ (with pkgs.xorg;
-    [ libX11
-      libXcursor
-      libXrandr
-    ]);
-  multiPkgs = pkgs: (with pkgs;
-    [ udev
-      alsa-lib
-    ]);
+  targetPkgs = pkgs: (with pkgs; [
+    udev
+    alsa-lib
+  ]) ++ (with pkgs.xorg; [
+    libX11
+    libXcursor
+    libXrandr
+  ]);
+  multiPkgs = pkgs: (with pkgs; [
+    udev
+    alsa-lib
+  ]);
   runScript = "bash";
 }).env
 ```