Update index.md
Update index.md

fix typo

Former-commit-id: bf11edf449802cd09433693214ddd1b6db59966c [formerly afbed3abd84f42d42fee186c2b429b27accdb8c2]
Former-commit-id: 372a8390eae4a78869cd3bcb85b3e40ea75b121f
LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io