intro: use actual predictor and outcome names in explanatory plots

[simonpcouch]

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For the logistic regression and decision tree explanations in Deck 3, we should use forested/non-forested and an example predictor in axis titles rather than one/two and A, perhaps noting in an aside that the data are simulated. In a similar situation later on, I remember trying to explain that connection and getting some confused looks.

https://github.com/tidymodels/workshops/blob/5d4486a24144c73fd8df17fc7bbf58035e2ae684/slides/intro-03-what-makes-a-model.qmd#L211-L236