[Tails-l10n] Heads up: check_po.sh has moved

Hi,

the check_po script has moved to a Git submodule (#9567) to avoid code
duplication => please read:

https://tails.boum.org/contribute/git/#submodules

I've also updated the corresponding piece of doc accordingly:

https://tails.boum.org/contribute/l10n_tricks/#check-po

Cheers,
--
intrigeri