chore(standard-integration): update fetch calls to match public docs

[gregjopa]

This PR makes the following changes to the standard-integration example:

1. Updates the api paths to match the docs(/my-server/create-paypal-order and /my-server/capture-paypal-order)
2. Updates the fetch calls to set the content-type as application/json
3. Adds the built in express.json() middleware to parse the post body.