Date "countdown" example does not match the actual style

When running `task columns` you see output like

```
start date Modifiable formatted* 2025-12-26
julian 2461036.34887
epoch 1766780542
iso 20251226T202222Z
age 2min
relative -2min
remaining
countdown 0:02:05
active *
```

But in fact, the `countdown` style is implemented to be identical to `relative` ~except that `age` erases its sign~ except that it is blank when `relative` would be negative (prior to #2542 `countdown` was also negated but this was considered a bug). The `0:02:05` format described in the example text is implemented as `Duration::format` but amusingly is not called anywhere in the codebase except in the code that generates this example text.

This change was made in 995de68c903707a45c30e6e7b4ff124537917cf5 in Aug 2015 which was part of the long-running "isodate" branch. My suspicion is that changing the "countdown" style was unintentional since this was always supposed to be an ad-hoc format rather than a standard one.

I propose we change `countdown` to match the example text.