Update data directories to match documentation
A code example on doc/advanced/external-gitaly/external-omnibus-gitaly.md
is supposed to be kept in sync with information shown at https://docs.gitlab.com/ee/administration/gitaly/index.html, but is incorrect per @marcel.amirault.
I'm attempting to update it to match.
Please update the info if it's wrong!
Edited by 🤖 GitLab Bot 🤖