Added availability zone to event scope

"availability zone" already added in the Linux's document.

https://docs.microsoft.com/en-us/azure/virtual-machines/linux/scheduled-events
> All the VMs in an availability set/ availability zone.

Ref: https://github.com/MicrosoftDocs/azure-docs/issues/55231

Author: tksh164

articles/virtual-machines/windows/scheduled-events.md

@@ -132,7 +132,7 @@ Each event is scheduled a minimum amount of time in the future based on event ty
 Scheduled events are delivered to:
  - Standalone Virtual Machines
  - All Virtual Machines in a Cloud Service		
- - All Virtual Machines in an Availability Set		
+ - All Virtual Machines in an Availability Set/Availability Zone
  - All Virtual Machines in a Scale Set Placement Group. 		
 
 As a result, you should check the `Resources` field in the event to identify which VMs are going to be impacted.