scrapy: `item_urls` is not defined in example

Hi @FannyVieira - I think the comment on line 21, and other comments like it, are supposed to be placeholders for code-that’s-not-included-in-this-demo. So item_urls is simply assumed to be parsed somehow from the response, but we don’t show how that happened.

This isn’t clear to everyone, so either we should:

• Include a minimal-but-complete example where all variables are extracted and defined fully
• Change the way we present “stuff happened, but we didn’t show you” in the examples. Maybe show the variables being defined by a method call, but the method-body is commented as # < processing code not shown > to be super clear.

By the way, are you interested in GSoC 2018? 😃