Update MaxMind-DB-spec.md

Few corrections:
 - The /80 in ::ffff:0:0/80 would cut off the ffff part.
 - uint128 examples don't subtract 7.
 - 29 + 51 = 80
Merge pull request #4 from kloesing/patch-1

Update MaxMind-DB-spec.md
Merge branch 'master' into gh-pages