Update iothub.json (#22252)

Update details on fallback properties

Author: Usha-Rathnavel

specification/iothub/resource-manager/Microsoft.Devices/stable/2021-07-02/iothub.json

@@ -2875,7 +2875,7 @@
           ]
         },
         "fallbackRoute": {
-          "description": "The properties of the route that is used as a fall-back route when none of the conditions specified in the 'routes' section are met. This is an optional parameter. When this property is not set, the messages which do not meet any of the conditions specified in the 'routes' section get routed to the built-in eventhub endpoint.",
+          "description": "The properties of the route that is used as a fall-back route when none of the conditions specified in the 'routes' section are met. This is an optional parameter. When this property is not present in the template, the fallback route is disabled by default.",
           "$ref": "#/definitions/FallbackRouteProperties"
         },
         "enrichments": {