Fix Timex.Duration.zero/0 documentation example

[nwjlyons]

Summary of changes

I'll review the commits, so I mostly want to understand the "why" rather than the "what"

Checklist

• [ ] New functions have typespecs, changed functions were updated
• [ ] Same for documentation, including moduledocs
• [ ] Tests were added or updated to cover changes
• [ ] Commits were squashed into a single coherent commit
• [ ] Notes added to CHANGELOG file which describe changes at a high-level