Fix pt_BR plural form expression.

The expressions for Portuguese and Brazilian Portuguese were incorrectly the same, but there's a subtle difference: in `pt`, 0 is considered plural, while it is singular in `pt_BR`.

Sources: `gettext-tools/src/plural-table.c` in GNU gettext 0.18.3.1 as well as Mozilla and Launchpad language descriptions:
https://translations.launchpad.net/+languages/pt_BR
https://translations.launchpad.net/+languages/pt
(Unicode CLDR doesn't have a Brazilian variant entry)