Update deviceupdate.json

Author: visingl

specification/deviceupdate/resource-manager/Microsoft.DeviceUpdate/preview/2020-03-01-preview/deviceupdate.json

@@ -1118,6 +1118,7 @@
         },
         "connectionString": {
           "description": "ConnectionString of the diagnostic storage account",
+          "x-ms-secret": true,
           "type": "string"
         },
         "resourceId": {