Update the documentation to setup palyload

Problem/Motivation

I was following the documentaion from the project page how to use the password grant but it didn't work until I also pass the scope.

{
  "grant_type": "password",
  "client_id": "__your-client-id__",
  "client_secret": "__your-client-secret__",
  "username": "drupal_username_or_email",
  "password": "drupal_password"
}

Proposed resolution

Update the documentation to also pass the scope in the payload. For example:

{
  "grant_type": "password",
  "client_id": "__your-client-id__",
  "client_secret": "__your-client-secret__",
  "username": "drupal_username_or_email",
  "password": "drupal_password",
  "scope": "__scope-name__",
}