Spelling: correct spelling of "passphrase".

There were several different variant spellings of "passphrase". This
should fix them all for all English text.

I did notice that po/it.po contains multiple instances of
"passhprase", which also looks suspect to me, but i do not know
Italian, so i did not try to correct it.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>