Document format for `--since` and `--until` flags on `fly builds`

[matthewpereira]

The documentation for fly builds describes a --since flag, but doesn't specify the date format to provide.

According to @YoussB the format should be yyyy-mm-dd HH:mm:ss. It might be a good idea to update the documentation page for fly builds or amend the format template to the fly builds --help output for since/until flags.