doc/design: fix keys.data MAC format description
doc/design: fix keys.data MAC format description

"not including the last 32 byte" was wrong, should have been 16 bytes. But the whole description is redundant anyway.
middelink/restic,kurin/restic,fawick/restic,middelink/restic,ar-jan/restic,kurin/restic,fawick/restic,restic/restic,restic/restic,ar-jan/restic