Update FOSUserBundle.pl.yml

Added translation for the commented-out key registration.back (which is translated for other languages).
minor #1682 Update Polish translations (XanderEmu)

This PR was submitted for the master branch but it was merged into the 1.3.x branch instead (closes #1682).

Discussion
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Update Polish translations

Added translation for the commented-out key registration.back (which is translated for other languages).

Commits
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294ab48 Update FOSUserBundle.pl.yml