Update trigger configuration documentation to show how to use appsettings

It is not clear from documentation on how to use AppSettings instead of string literals while configuring a trigger. Following is the eventhub trigger documentation:
[functions-bindings-event-hubs#trigger---configuration](https://docs.microsoft.com/en-us/azure/azure-functions/functions-bindings-event-hubs#trigger---configuration)

Needs to include

path | EventHubName | The name of the event hub. Can be referenced via app settings %eventHubName%

Two CRIs were created due to confusion on how to set these values via appsettings