Update merkle.cpp
Update merkle.cpp

Change comment from `The reason is that if the number of hashes in the list at a given time
is odd`, to ` The reason is that if the number of hashes in the list at a given level
       is odd` (to be a bit more precise)
peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin