docs: fix highlighting in code block

clanServices/users/README.md

@@ -1,11 +1,10 @@
 ## Usage
 
-```
-inventory.instances = {
-
+```nix
+{
+  inventory.instances = {
     # Deploy user alice on all machines. Don't prompt for password (will be
     # auto-generated).
-
     user-alice = {
       module = {
         name = "users";
@@ -27,4 +26,6 @@ inventory.instances = {
       roles.default.machines.bobs-laptop = { };
       roles.default.settings.user = "bob";
     };
+  };
+}
 ```