maintainer-pgp-guide: use key terminology consistent with upstream

GnuPG does not use the word "master key" when referring to the subkey
marked with the "certification" capability. Our use of this term was not
only inconsistent, but also misleading, because in real life "master
keys" are able to open multiple locks made for different keys, while PGP
Certify key has no such capability.
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220727-docs-pgp-guide-v2-1-e3e6954affb6@linuxfoundation.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>