Fix type parameter in change_savings_plan requests

[SimDing]

Either this has changed, or this is just a copy/paste error.

With type createSavingsPlan I was getting these errors:

{
  "status": "failed",
  "message": "Could not create savings plan",
  "error": {
    "code": "internalError",
    "message": "Could not create savings plan",
    "details": {
      "userId": "...",
      "requestedAmount": 42.0
    }
  }
}