message: Fix interface version in comment
diff: //! exports exposes the public wasm API
  //!
  -//! interface_version_7, allocate and deallocate turn into Wasm exports
  +//! interface_version_8, allocate and deallocate turn into Wasm exports
  //! as soon as cosmwasm_std is `use`d in the contract, even privately.
  //!
  //! `do_execute`, `do_instantiate`, `do_migrate`, `do_query`, `do_reply`
  