Update custom-code.md

docs/custom-code.md

@@ -147,15 +147,15 @@ oc.thread.on("MessageAdded", async function() {
 
 ### "<function> is not defined" in click/event handlers
 The following code won't work:
-```
+```js
 function hello() {
   console.log("hi");
 }
 document.body.innerHTML = `<div onclick="hello()">click me</div>`;
 oc.window.show();
 ```
 This is because all custom code is executed inside a &lt;script type=module&gt; so you need to make functions *global* if you want to access them from *outside* the module (e.g. in click handlers). So if you want to the above code to work, you should define the `hello` function like this instead:
-```
+```js
 window.hello = function() {
   console.log("hi");
 }