fr: normalize with mdbook-i18n-helpers 0.2.2 (#1107)

Before, po/fr.po had these statistics:

312 translated messages, 984 fuzzy translations, 485 untranslated
messages.

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

343 translated messages, 1363 fuzzy translations, 761 untranslated
messages.

The number of translated messages changed from 18% to 14%.

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.