Missing User-Agent header in Wikipedia scraping example

Hi! The web scraping example in Section 15 that fetches a Wikipedia page seems not to work anymore, because Wikipedia now rejects requests without a User-Agent header. Instead, it returns a message "Please set a user-agent and respect our robot policy https://w.wiki/4wJS.".

I would be happy to submit a small PR that adds a User-Agent header to the example notebook.