[c++] std::shared_ptr circular reference example incorrect?

https://learnxinyminutes.com/docs/c++/

I do not think the example creates or exemplifies circular pointers.

```
// Beware of possible circular references!!!
// There will be always a reference, so it will be never destroyed!
std::shared_ptr doggo_one(new Dog());
std::shared_ptr doggo_two(new Dog());
doggo_one = doggo_two; // p1 references p2
doggo_two = doggo_one; // p2 references p1
```
The `doggo_one`/`doggo_two` manipulation just looks like it changes which `Dog` instance the smart pointers refers to, without creating any cycles.

I do not know C++ (that's why I'm here) but my guess is that circular (smart) pointers would require, e.g., that each `Dog` object to contain a `std::shared_ptr` to the other `Dog` object.

Start from the C++ page linked in the issue and inspect the shown std::shared_ptr circular-reference example. Verify whether the two pointer assignments actually create a cycle, then update the documentation so the warning and example match. Done means the C++ docs no longer present an incorrect circular reference example.

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