201-alert-to-slack-with-logic-app

[201-alert-to-slack-with-logic-app](https://github.com/Azure/azure-quickstart-templates/tree/master/201-alert-to-slack-with-logic-app)

### Issue Details
Readme.MD instructions on how to use the template with the Portal are incorrect.
[Call from your Alerts](https://github.com/Azure/azure-quickstart-templates/tree/master/201-alert-to-slack-with-logic-app#call-from-your-alerts) section:
Steps 2, 3 and 4 do not accurately reflect what is shown in the Azure portal for a Logic App.

There is no **last deployment** link in Essentials
there is no _top deployment_ to choose
there is no output called **WebHookURI**

### Repro steps
1. Deploy template
2. try to follow the steps as listed in the Readme.MD
3. you'll find multiple errors

I'm willing to fix them, but I'm not sure what is the correct options.

Open the 201-alert-to-slack-with-logic-app Readme.MD and review the “Call from your Alerts” section, especially steps 2–4. Compare those instructions with the current Azure Portal Logic App interface and verify the deployment and WebHookURI references. Done means the README gives accurate, reproducible portal steps.

Written by the indexing model from the issue text.