Updating the doc with permanent assignment steps

We talk about schedule here but do not explain how to do permanent assignment. Added that here.

Ref:https://docs.microsoft.com/en-us/graph/api/resources/governanceschedule?view=graph-rest-beta#properties

Author: ManojReddy-MSFT

articles/active-directory/privileged-identity-management/powershell-for-azure-ad-roles.md

@@ -93,6 +93,8 @@ The schedule, which defines the start and end time of the assignment, is an obje
     $schedule.Type = "Once"
     $schedule.StartDateTime = (Get-Date).ToUniversalTime().ToString("yyyy-MM-ddTHH:mm:ss.fffZ")
     $schedule.endDateTime = "2020-07-25T20:49:11.770Z"
+> [!Note]
+> If the value of endDateTime is set to null, it indicates a permanent assignment.
 
 ## Activate a role assignment