[typo] Fix syntax error on properties.md (#1267)

Author: louwers

packages/lit-dev-content/site/docs/v3/components/properties.md

@@ -54,7 +54,7 @@ private _counter = 0;
 
 ```js
 static properties = {
-  _counter: {state: true};
+  _counter: {state: true}
 };
 
 constructor() {