Update IsCustom property on example role defintion (Azure#21514)

* Update IsCustom property on example role defintion

We're making a custom role definition, so we need to update the IsCustom property to True.

* Update New-AzRoleDefinition.md

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Co-authored-by: Yunchi Wang <54880216+wyunchi-ms@users.noreply.github.com>

Author: geologyrocks

src/Resources/Resources/help/New-AzRoleDefinition.md

@@ -87,6 +87,7 @@ $role = Get-AzRoleDefinition -Name "Virtual Machine Contributor"
 $role.Id = $null
 $role.Name = "Virtual Machine Operator"
 $role.Description = "Can monitor, start, and restart virtual machines."
+$role.IsCustom = $True
 $role.Actions.RemoveRange(0,$role.Actions.Count)
 $role.Actions.Add("Microsoft.Compute/*/read")
 $role.Actions.Add("Microsoft.Compute/virtualMachines/start/action")