drivers: rtc: rtc_shell: fix milliseconds display

The RTC shell get command displays the time according to the ISO8601
standard. Fix the milliseconds field to be displayed on 3 digits instead
of 6 like if they were microseconds. In addition change the separator
between seconds and milliseconds to a dot like specified in the ISO8601
standard.

Before:
2024-06-02T22:33:10:000667

After:
2024-06-02T22:33:10.667

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>