Fix PowerShell cmdlet name

Update-AzVmssInstances cmdlet is not exists. The last "s" is unnecessary.

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cc: @ju-shim

Author: tksh164

articles/virtual-machine-scale-sets/virtual-machine-scale-sets-health-extension.md

@@ -329,7 +329,7 @@ Update-AzVmss -ResourceGroupName $vmScaleSetResourceGroup `
   -VirtualMachineScaleSet $vmScaleSet
   
 # Upgrade instances to install the extension
-Update-AzVmssInstances -ResourceGroupName $vmScaleSetResourceGroup `
+Update-AzVmssInstance -ResourceGroupName $vmScaleSetResourceGroup `
   -VMScaleSetName $vmScaleSetName `
   -InstanceId '*'
 ```
@@ -455,7 +455,7 @@ Update-AzVmss -ResourceGroupName $vmScaleSetResourceGroup `
   -VirtualMachineScaleSet $vmScaleSet
 
 # Upgrade instances to install the extension
-Update-AzVmssInstances -ResourceGroupName $vmScaleSetResourceGroup `
+Update-AzVmssInstance -ResourceGroupName $vmScaleSetResourceGroup `
   -VMScaleSetName $vmScaleSetName `
   -InstanceId '*'
 ```