[Tails-dev] [doc] signing key expriy date needs <3

Hi,

I just saw that the expiry date of the signing key on
https://tails.boum.org/doc/about/openpgp_keys/index.en.html needs to be
updated as it expired 2 days ago.

I am pretty busy today, so I can't commit to do that right now, but if
nobody else works on it, I can take care of it at a later point in time.

Cheers
u.