example_18_06_XMLYahooWeather out of date

[ybakos]

The Yahoo weather API has recently changed, and this example needs a little massaging.
Ref: processing/processing#4422

Will send a minimal PR that shouldn't require too much change to the text. Biggest difference is the need to introduce the concept of a "woeid".

[shiffman]

Thanks so much for this fix! I'm going to re-open this issue as I'll need to change the book text itself as well as some point. (In a different repo that I am not allowed to make public as much as I have tried.)