environment variable API

[yxiao168]

It is confusing about the value of environment variable API in
"https://github.com/MicrosoftDocs/mslearn-aks-workshop-ratings-web" and "https://github.com/MicrosoftDocs/mslearn-aks-workshop-ratings-api".

Suggest to change the sentence "API requests via the backend to the rating-api endpoint" and use "http://localhost:3000/healthz" to check the API health before running this application.

API requests via the backend to the rating-api endpoint. e.g., 
$ export API="http://localhost:3000"

$ curl -XGET http://localhost:3000/healthz;echo
API health check - OK