ko: normalize with mdbook-i18n-helpers 0.2.2

Before, po/ko.po had these statistics:

    1323 translated messages, 231 fuzzy translations, 227 untranslated messages.

Afterwards, the statistics for po/ko.po is:

    1898 translated messages, 259 fuzzy translations, 310 untranslated messages.

The number of translated messages changed from 74% to 77%.

With this change, it becomes important to use the latest version of
mdbook-i18n-helpers when viewing the translation locally. To update to
the latest version, run

    cargo install mdbook-i18n-helpers

You will now be able to serve the translation locally.

Part of #330.