treewide: Fix all Nix ASTs in all markdown files

This allows for correct highlighting and maybe future automatic
formatting. The AST was verified to work with nixfmt only.

nixos/modules/services/development/athens.md

@@ -42,7 +42,7 @@ To use the local proxy in Go builds, you can set the proxy as environment variab
 ```nix
 {
   environment.variables = {
-    GOPROXY = "http://localhost:3000"
+    GOPROXY = "http://localhost:3000";
   };
 }
 ```

nixos/modules/services/development/livebook.md

@@ -52,5 +52,7 @@ learning Kinos require `gcc` and `gnumake`.  To add these, use
 `extraPackages`:
 
 ```nix
-services.livebook.extraPackages = with pkgs; [ gcc gnumake ];
+{
+  services.livebook.extraPackages = with pkgs; [ gcc gnumake ];
+}
 ```