fa: normalize with mdbook-i18n-helpers 0.2.2 (#1108)

Before, po/fa.po had these statistics:

300 translated messages, 4 fuzzy translations, 1477 untranslated
messages.

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

291 translated messages, 6 fuzzy translations, 2170 untranslated
messages.

The number of translated messages changed from 17% to 12%.

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.