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)

Former-commit-id: 6f42d5b54bd8e07fe38964191ef22cc9d5e5bf01
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin