Customizing touch bar word predictions
Assuming it to be same as the one in keyboard predictions on iOS, Apple would have put poor Siri's name for this tool too. The best way I found to train it is to use it multiple times. For a persistent error, text replacement to the rescue.

Where does Apple's predictive keyboard get its “contextual” content from?

This question is for iOS, but it's not unreasonable to assume that prediction algorithm can be same in both places.