Fix BYIOP spectrum link and add example snippet

[Nondv]

Summary

the current link is broken (see https://developers.cloudflare.com/byoip/address-maps/).

To be honest, I imagine this kind of links is simply prone to breaking. But at least this will fix it for now

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X. ANSWER: it's just a link fix
• The change adheres to the documentation style guide.