zh-TW: normalize with mdbook-i18n-helpers 0.2.2

Before, po/zh-TW.po had these statistics:

    492 translated messages, 55 fuzzy translations, 1234 untranslated messages.

Afterwards, the statistics for po/zh-TW.po is:

    776 translated messages, 67 fuzzy translations, 1624 untranslated messages.

The number of translated messages changed from 28% to 31%.

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.