[Tails-l10n] [fr](review) signing_key_revocation

Hello,

Please review the translation of the signing key revocation process.
I couldn't manage to build the "goals" section while keeping the
formatting. It looks like a plain bloc of text on my setup.

repo: https://gitlab.com/AtomiKe/tails
branch: signing_key_revocation
commit: 04ffeb19ef7ca0421fc33128c4857f0a6d053ab2

Cheers,
AtomiKe.

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