Fix the Logger::Formatter documentation

[liijunwei]

Fix #78

[liijunwei]

Logger::Formatter::DatetimeFormat is "%Y-%m-%dT%H:%M:%S.%6N" and it doesn't include Timezone info.

If timezone info is required , we better add %Z formatter

e.g.

Time.now.strftime("%Y-%m-%dT%H:%M:%S.%6N %Z")
=> "2022-08-20T23:15:47.742414 CST"