API Spec has invalid model name

The "rate estimate" model in the API Spec has an invalid model name, it should be "rate_estimate" instead. This is preventing auto generating API clients.

https://github.com/ShipEngine/shipengine-openapi/blob/master/openapi.yaml#L9258
```
    rate estimate:
      title: rate estimate
      type: object
      description: A rate estimate
      required:
```