Chapter 2 exercise answer bug

The exercise answer could not handle negative values:

```cpp
template<typename ... T>
auto average(T ... t) {
    return (t + ... ) / sizeof...(t);
}
int main() {
    std::cout << average(1, 2, 3, 4, 5, -6, -7, -8, -9, -10) << std::endl;
}
```

It would return `1844674407370955159` on my side since it deduced the return value to be unsigned int.