Pre-configured Stitch To-Do App expects cluster name StitchDemoCluster

Description

Link: https://docs.mongodb.com/stitch/tutorials/todo-web/#a-create-or-import-a-new-stitch-short-app

Following the steps with an already existing Atlas cluster will result in error:

failed to import app: error: cluster 'StitchDemoCluster' not found: No cluster with ID StitchDemoCluster exists in group ...

This can be fixed by adding actual cluster name to todo/import/Todo_Web_App_With_Twilio/services/mongodb-atlas/config.json.

So, we should add a step in the docs to ensure that the correct cluster name is used in this config file.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)