Update lmdb++.h

[Smorodov]

DB folder was not created, while opening DB in windows.
env.open(DB_name.c_str()); in example program exited after this line with code 3.

[davidalbertonogueira]

I fixed it with:

env& open(const char* const path, const unsigned int flags = default_flags) {
  if (!(flags & MDB_NOSUBDIR))
    fs::create_directory(path);
  lmdb::env_open(handle(), path, flags, default_mode);
  return *this;
}