Custom Adjustment example requires `calculate()` method

The example custom adjustment on [this page](https://github.com/littleweaver/django-daguerre/blob/6d27bc061b10a4433b3ab25671ac2c0714692c5d/docs/api/adjustments.rst) doesn't have a `calculate()` method, but the documentation states that all `Adjustment` subclasses must have a `calculate()` method, so we should probably add one that just returns the original dimensions.
