[locale] ko: Add trailing dot to L and l

[marwahaha]

fixes #4070

[marwahaha]

@YenalTuran - is the trailing dot ever not present?

[YenalTuran]

@marwahaha it is present only after year and month, but not day.

[marwahaha]

@YenalTuran

• how often do you use the format 2017.12.29 ?
• how often do you use the format 2017.12.29. ?

If you use both, then I will only change L and not l.

[YenalTuran]

@marwahaha currently we show our Korean customers their logistics' booking dates in 2017.12.29 format, but since the correct format is 2017.12.29. I would like to use moment library (without any workaround) and show the expected format :) many thanks for your support!