Update index.md

[kugelzucker]

typo

Prerequisites checklist

• [x] I have read the contributing guidelines.

What is the purpose of this pull request? (put an "x" next to an item)

• [x] Documentation update
• [ ] Bug fix
• [ ] New feature
• [ ] Other, please explain:

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?