pl: normalize with mdbook-i18n-helpers 0.2.2 (google#1103)

Before, po/pl.po had these statistics:

441 translated messages, 713 fuzzy translations, 627 untranslated
messages.

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

666 translated messages, 882 fuzzy translations, 919 untranslated
messages.

The number of translated messages changed from 25% to 27%.

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.