chore: improve service worker section (#766)

Minor improvements to the service workers section of the theme docs

Author: davestaab

docs/default-theme-config/README.md

@@ -358,15 +358,15 @@ Note that it's `off` by default. If given a `string`, it will be displayed as a
 
 ## Service Worker
 
-The `themeConfig.serviceWorker` option allows you to configure about service worker.
+The `themeConfig.serviceWorker` option allows you to configure the service worker.
 
 ::: tip
 Please do not confuse this option with [Config > serviceWorker](../config/README.md#serviceworker), [Config > serviceWorker](../config/README.md#serviceworker) is **site-level**, while this option is **theme-level**.
 :::
 
 ### Popup UI to refresh contents <Badge text="0.13.0+"/> <Badge text="beta" type="warn"/>
 
-The `themeConfig.serviceWorker.updatePopup` option enables the popup to refresh contents. The popup will be shown when the site is updated (i.e. service worker is updated). It provides `refresh` button to allow users to refresh contents immediately.
+The `themeConfig.serviceWorker.updatePopup` option enables a popup to refresh site content. The popup will be shown when the site is updated (i.e. service worker is updated). It provides a `refresh` button to allow users to refresh contents immediately.
 
 ::: tip NOTE
 If without the `refresh` button, the new service worker will be active after all [clients](https://developer.mozilla.org/en-US/docs/Web/API/Clients) are closed. This means that visitors cannot see new contents until they close all tabs of your site. But the `refresh` button activates the new service worker immediately.