bad variable name in example

It should be the same as the first example that gets the VM.
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cc: @cynthn

Author: mgreenegit

articles/virtual-machine-scale-sets/tutorial-use-custom-image-powershell.md

@@ -115,7 +115,7 @@ New-AzGalleryImageVersion `
    -ResourceGroupName $resourceGroup.ResourceGroupName `
    -Location $resourceGroup.Location `
    -TargetRegion $targetRegions  `
-   -Source $vm.Id.ToString() `
+   -Source $sourceVM.Id.ToString() `
    -PublishingProfileEndOfLifeDate '2020-12-01'
 ```