Update index.md

[nickbyfleet]

Changed a sentence so that it makes sense.

Thanks for contributing, you're awesome! ⭐
Please describe expected and observed behaviour, and what you're fixing.
For visual fixes, please include tested browsers and screenshots.
Search for related existing issues and link to them if possible.
Please read https://docs.silverstripe.org/en/contributing/code/

[robbieaverill]

Thank you!