Half-hour negative timezone formatting

### Steps to reproduce

Call `Timex.TimezoneInfo.format_offset/1` with a half-hour negative timezone, e.g. `UTC-9:30`.

```
%Timex.TimezoneInfo{abbreviation: "-0930", full_name: "Pacific/Marquesas", offset_std: 0, offset_utc: -34200}
|> Timex.TimezoneInfo.format_offset()
# "-09:-30:00"
```

### Description of issue

Formatting half-hour negative timezones returns `"-09:-30:00"`, and I was expecting `"-09:30:00"`, without that additional minute minus sign. I may be wrong, but is that the conventional formatting for offsets like that?