Document `%e` format token

> Writes the day of month as a decimal number. If the result is a single decimal digit, it is prefixed with a space.

https://en.cppreference.com/w/cpp/chrono/zoned_time/formatter

```r
> clock::date_format(as.Date("2020-01-02"), format = "%Y-%m-%e")
[1] "2020-01- 2"
```