pt-BR: normalize with mdbook-i18n-helpers 0.2.2 (#1104)

Before, po/pt-BR.po had these statistics:

1272 translated messages, 93 fuzzy translations, 416 untranslated
messages.

Afterwards, the statistics for po/pt-BR.po is:

1703 translated messages, 114 fuzzy translations, 650 untranslated
messages.

The number of translated messages changed from 71% to 69%.

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.