Update introduction.md

[tommasocerruti]

Add an example also for the DateTime module, making it analogous to the other modules.

[github-actions[bot]]

Thank you for contributing to exercism/elixir 💜 🎉. This is an automated PR comment 🤖 for the maintainers of this repository that helps with the PR review process. You can safely ignore it and wait for a maintainer to review your changes.

Based on the files changed in this PR, it would be good to pay attention to the following details when reviewing the PR:

• General steps

  • 🏆 Does this PR need to receive a label with a reputation modifier (x:size/{tiny,small,medium,large,massive})? (A medium reputation amount is awarded by default, see docs)

• Concept changed

  • 👤 Does the author of the PR need to be added as an author or contributor in <concept>/.meta/config.json (see docs)?
  • 📖 Does the concept introduction provide all necessary information to solve this exercise?

---

Automated comment created by PR Commenter 🤖.

[angelikatyborska]

Thank you, but we omitted the code sample or the mention of the DateTime's sigil on purpose.

Timezones are out of scope for this exercise because they always require an external dependency with a database of timezones. And we don't use external dependencies in any of the Elixir Exercism exercises.