[apt-doc: guide] s/GPGP/GPG/?

[victory <victory.deb@gmail.com>]

doc/guide.dbk line 77:
with GPG. Without GPGP installed mailcrypt is useless, so mailcrypt has a

this is very old string though,
maybe s/GPGP/GPG/ here?

-- 
victory
no need to CC me :-)
http://userscripts.org/scripts/show/102724 0.0.1.4
http://userscripts.org/scripts/show/163846 0.0.1
http://userscripts.org/scripts/show/163848 0.0.1