Fix codeowner syntax for docs

[samjwu]

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#codeowners-syntax

# The `docs/*` pattern will match files like
# `docs/getting-started.md` but not further nested files like
# `docs/build-app/troubleshooting.md`.
docs/*  docs@example.com