chore(docs): Improve "Adding a service worker" (#33737)

Co-authored-by: Lennart <[email protected]>

Author: Rutam21

docs/docs/add-a-service-worker.md

@@ -2,15 +2,15 @@
 title: Adding a Service Worker
 ---
 
-## What is a service worker
+## What is a service worker?
 
-A service worker is a script that your browser runs in the background, separate from a web page, opening the door to features that don't need a web page or user interaction. They increase your site availability in spotty connections, and are essential to making a nice user experience.
+A service worker is a script that your browser runs in the background, separate from a web page, opening the door to features that don't need a web page or user interaction. They increase your site availability in spotty connections and are essential to making a nice user experience.
 
 It supports features like push notifications and background sync.
 
 ## Using service workers in Gatsby with `gatsby-plugin-offline`
 
-Gatsby provides awesome plugin interface to create and load a service worker into your site [gatsby-plugin-offline](https://www.npmjs.com/package/gatsby-plugin-offline).
+Gatsby provides an awesome plugin interface to create and load a service worker into your site: [gatsby-plugin-offline](https://www.npmjs.com/package/gatsby-plugin-offline).
 
 You can use this plugin together with the [manifest plugin](https://www.npmjs.com/package/gatsby-plugin-manifest). (Don’t forget to list the offline plugin after the manifest plugin so that the manifest file can be included in the service worker).