docs: Add example to guard-for-in docs. (#16983)

Adds an example to the correct code for the guard-for-in rule
that uses Object.hasOwn

Fixes #16981

docs/src/rules/guard-for-in.md

@@ -44,6 +44,12 @@ Examples of **correct** code for this rule:
 ```js
 /*eslint guard-for-in: "error"*/
 
+for (key in foo) {
+    if (Object.hasOwn(foo, key)) {
+        doSomething(key);
+    }
+}
+
 for (key in foo) {
     if (Object.prototype.hasOwnProperty.call(foo, key)) {
         doSomething(key);