minor change in RoutingTable::NthElementSortFromTarget

minor change in RoutingTable::NthElementSortFromTarget
```
int main() {
  std::vector<int> numbers, copy;
  for (int index(0); index < 10; ++index)
    numbers.push_back(index);

  for (int index(0); index < 10; ++index) {
    std::random_shuffle(numbers.begin(), numbers.end());
    copy = numbers;
    std::partial_sort(numbers.begin(), numbers.begin() + index, numbers.end());
    std::nth_element(copy.begin(), copy.begin() + index, copy.end());  
    std::cout << "partial sorted at " << index << " is " << numbers.at(index) << std::endl;
    std::cout << "nth elemented at " << index << ", is " << copy.at(index) << std::endl;
    for (int index(0); index < 10; ++index)
      std::cout << numbers.at(index) << " ";
    std::cout << "Partial\n";
    for (int index(0); index < 10; ++index)
      std::cout << copy.at(index) << " ";
    std::cout << "Nth_elem \n\n";
  }
  return 0;
```