addEventListener: Update example function name to prevent minor confu…

…sion (mdn#30162)

The notes above this example teach that the second argument of the `addEventListener` function can accept an object that has a `handleEvent()` function. The example then shows what that might look like but names the function `eventHandler`.

This can cause a little bit of confusion so this PR just makes a minor rename to mitigate that.

files/en-us/web/api/eventtarget/addeventlistener/index.md

@@ -122,7 +122,7 @@ For example, an event handler callback that can be used to handle both
 {{domxref("Element/fullscreenerror_event", "fullscreenerror")}} might look like this:
 
 ```js
-function eventHandler(event) {
+function handleEvent(event) {
   if (event.type === "fullscreenchange") {
     /* handle a full screen toggle */
   } else {