{Compute} Update help of az vmss disk attach (#12677)

Author: qwordy

src/azure-cli/azure/cli/command_modules/vm/_help.py

@@ -2124,7 +2124,7 @@
     crafted: true
   - name: Attach managed data disks of a given size to a scale set or its instances. (autogenerated)
     text: |
-        az vmss disk attach --name MyVmss --resource-group MyResourceGroup --size-gb 50
+        az vmss disk attach --vmss-name MyVmss --resource-group MyResourceGroup --size-gb 50
     crafted: true
 """