Update Set-AzureRmVMBootDiagnostics.md

add Update-AzureRmVM

[related docs]
https://docs.microsoft.com/en-us/azure/virtual-machines/windows/troubleshoot-recovery-disks#re-enable-boot-diagnostics

Author: ShuheiUda

src/ResourceManager/Compute/Stack/Commands.Compute/help/Set-AzureRmVMBootDiagnostics.md

@@ -32,6 +32,7 @@ The **Set-AzureRmVMBootDiagnostics** cmdlet modifies boot diagnostics properties
 ```
 PS C:\> $VM = Get-AzureRmVM -ResourceGroupName "ResourceGroup11" -Name "ContosoVM07"
 PS C:\> Set-AzureRmVMBootDiagnostics -VM $VM -Enable -ResourceGroupName "ResourceGroup11" -StorageAccountName "DiagnosticStorage"
+PS C:\> Update-AzureRmVM -ResourceGroup "ResourceGroup11" -VM $VM
 ```
 
 The first command gets the virtual machine named ContosoVM07 by using **Get-AzureRmVM**.