message: update reference to app.py
diff: @@ -88,7 +88,7 @@ You can see that these two diagrams aren't a massive distance away from my high
  I separated each of the different SNS subscriber flows above into their own CDK stacks and passed in the SNS Topic ARN as a parameter. Note, in a production system if you want to properly separate these stacks you could use [AWS Systems Manager Parameter Store](https://docs.aws.amazon.com/systems-manager/latest/userguide/systems-manager-parameter-store.html) for the SNS Topic ARN.
  -if you look inside bin/the-xray-tracer.ts you will see how this works:
  +if you look inside app.py you will see how this works:
  ```python
  xray_tracer = TheXrayTracerStack(app, "the-xray-tracer")
  