uk: normalize with mdbook-i18n-helpers 0.2.2

Before, po/uk.po had these statistics:

    63 translated messages, 15 fuzzy translations, 1703 untranslated messages.

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

    104 translated messages, 19 fuzzy translations, 2344 untranslated messages.

The number of translated messages stayed at 4%.

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.