[w3c/ServiceWorker] Make cache example easier to understand (#976)

I personally think that:

```js
return cachedResponse || fetch(event.request);
```
 makes it a lot more obvious what's going on as opposed to:

```js
return response || fetch(event.request);
```

I had to do a double take with `response` as at first glance it looked like the request was being made twice.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/976

-- Commit Summary --

  * Make cache example easier to understand

-- File Changes --

    M explainer.md (4)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/976.patch
https://github.com/w3c/ServiceWorker/pull/976.diff

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