Clarify get_dummies behavior
Clarify get_dummies behavior

I was mystified by this `get_dummies` call:

1. "Dummies" means nothing to me; I didn't know it was another term for one-hot. I hinted at this by mentioning the function in the text.
2. How on earth does `get_dummies` know that `Origin` is the categorical property? Why not `Cylinders` also? It turns out the current code kinda "works by accident" - it's because the dtype of the Origin column is "object". If it was "string", it wouldn't work. I fixed this by being explicit about which column it should convert to one-hot.
tensorflow/docs,tensorflow/docs,tensorflow/docs