Refactor classification example

The vignette beginning at https://github.com/cmu-delphi/epipredict/blob/f3f5f19546e8ddce3adc22b69420dee6a6a8db31/vignettes/preprocessing-and-models.Rmd#L378
hand-codes a classifier. Now that `step_growth_rate()` is available (along with `arx_classifier()`), we should update the example to first show the use of the the growth rate calculation, and then simply call the classifier. We should be able to examine both and see the same results.