[Book 2]: I'm not sure if bvh_node's constructor is correct

[define-private-public]

https://raytracing.github.io/books/RayTracingTheNextWeek.html#boundingvolumehierarchies/thebvhnodeclass

I'm talking about this: bvh_node(std::vector<shared_ptr>& objects, size_t start, size_t end, double time0, double time1)`

In the definition, there is a sort() being performed on objects, which would modify the variable. I think the & should be removed.