Missing build step from readme, npm start will fail without this (#1735)

examples/aws-companion/readme.md

@@ -10,6 +10,7 @@ To run this example, make sure you've correctly installed the root repository:
 ```bash
 npm install
 npm run bootstrap
+npm run build
 ```
 
 Then, set up the `COMPANION_AWS_KEY`, `COMPANION_AWS_SECRET`, `COMPANION_AWS_REGION`, and `COMPANION_AWS_BUCKET` environment variables for @uppy/companion.