Fix grammatical issue in introduction.md

[bbenip]

Add missing preposition 'of'.

[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)

• Any exercise changed

  • 👤 Does the author of the PR need to be added as an author or contributor in <exercise>/.meta/config.json (see docs)?
  • 🔬 Do the analyzer and the analyzer comments exist for this exercise? Do they need to be changed?
  • 📜 Does the design file (<exercise>/.meta/design.md) need to be updated to document new implementation decisions?

• Concept exercise changed

  • 🌲 Do prerequisites and practices in config.json need to be updated?
  • 📖 Does the concept introduction provide all necessary information to solve this exercise?

• 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 🤖.