it: normalize with mdbook-i18n-helpers 0.2.2 (google#1105)

Before, po/it.po had these statistics:

447 translated messages, 927 fuzzy translations, 407 untranslated
messages.

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

583 translated messages, 1288 fuzzy translations, 596 untranslated
messages.

The number of translated messages changed from 25% to 24%.

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 google#330.