Updated Get-AzureRmDisk outputs section

Get-AzureRmDisk outputs an object of type Microsoft.Azure.Commands.Compute.Automation.Models.PSDiskList instead of a non-descriptive object. Updated the doc to clarify. You may validate this by running `(get-azurermdisk | select -first 1).gettype()`.

Author: thomasrayner

src/ResourceManager/Compute/Stack/Commands.Compute/help/Get-AzureRmDisk.md

@@ -82,7 +82,7 @@ This cmdlet supports the common parameters: -Debug, -ErrorAction, -ErrorVariable
 
 ## OUTPUTS
 
-### System.Object
+### Microsoft.Azure.Commands.Compute.Automation.Models.PSDiskList
 
 ## NOTES