[Docs] Add missing import in custom App code example (#10047)

* Updated comment

* Removed unexpected word

* Update custom-app.md

* Update custom-app.md
Co-authored-by: NJoe Haddad <timer150@gmail.com>

docs/advanced-features/custom-app.md

@@ -14,6 +14,8 @@ Next.js uses the `App` component to initialize pages. You can override it and co
 To override the default `App`, create the file `./pages/_app.js` as shown below:
 
 ```jsx
+// import App from 'next/app'
+
 function MyApp({ Component, pageProps }) {
   return <Component {...pageProps} />
 }