remove readonly attribute from customIpPrefix commissionedState property

Author: dexiang-ms

specification/network/resource-manager/Microsoft.Network/stable/2020-06-01/customIpPrefix.json

@@ -368,7 +368,6 @@
           "description": "The master custom IP Prefix parent."
         },
         "commissionedState": {
-          "readOnly": true,
           "type": "string",
           "description": "The commissioned state of the Custom IP Prefix.",
           "enum": [