Update instructions for complex-number exercise

[ignazioc]

• Introducing z in the definition of a complex number
• Formatting to have the code as separate blocks instead of inline with the rest of the text that made reading difficult.

[github-actions[bot]]

Hello. Thanks for opening a PR on Exercism. We are currently in a phase of our journey where we have paused community contributions to allow us to take a breather and redesign our community model. You can learn more in this blog post. As such, all issues and PRs in this repository are being automatically closed.

That doesn't mean we're not interested in your ideas, or that if you're stuck on something we don't want to help. The best place to discuss things is with our community on the Exercism Community Forum. You can use this link to copy this into a new topic there.

---

Note: If this PR has been pre-approved, please link back to this PR on the forum thread and a maintainer or staff member will reopen it.

[kotp]

Here is the link to the community forum post.

[junedev]

@ignazioc @kotp This exercise originates from https://github.com/exercism/problem-specifications from what I can see so fixes should be done there and then they are synced down.