Improve AJAX example [ci skip]

The example uses `fetch` to retrieve HTML from the server and append it to an element on the page. This commit updates the example to append the response HTML rather than the full response object.

guides/source/working_with_javascript_in_rails.md

@@ -44,7 +44,7 @@ fetch("/test")
   .then((data) => data.text())
   .then((html) => {
     const results = document.querySelector("#results");
-    results.insertAdjacentHTML("beforeend", data);
+    results.insertAdjacentHTML("beforeend", html);
   });
 ```