code example in README doesn't deploy

[ataraxia937]

Describe the bug
I used this library as shown in the README, and it didn't deploy because of a missing / invalid KMS key.

To Reproduce

1. Write code like this:

new SesSmtpCredentials(this, 'CsatSesSmtpCredentials', {
  iamUserName: 'csatmail',
});

2. Get error when deploying:

Stack Deployments Failed: Error: The stack named CsatStack failed creation, it may need to be manually deleted from the AWS console: ROLLBACK_COMPLETE: Received response status [FAILED] from custom resource. Message returned: Error: An error occurred (InvalidParameterException) when calling the CreateSecret operation: The operation failed because of an invalid KMS key: Invalid keyId aws/secretsmanager (Service: AWSKMS; Status Code: 400; Error Code: NotFoundException; Request ID: 7d8aa148-df6c-4f7b-9841-9a17dc591484; Proxy: null)

Expected behavior
Code like in the README should work. Or the README should be changed to reflect actual behavior

Additional context
This works:

const csatKey = new kms.Key(this, 'CsatKey');

new SesSmtpCredentials(this, 'CsatSesSmtpCredentials', {
  iamUserName: 'csatmail',
  kmsKey: csatKey,
});