R.13 is out of date with C++17

R.13 uses the example:

```
// BAD: potential leak
fun(shared_ptr<Widget>(new Widget(a, b)), shared_ptr<Widget>(new Widget(c, d)));
```

This could leak prior to C++17, but with the change from [this accepted proposal](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0145r3.pdf), there is no potential leak in C++17. The explanation of this rule should be updated to reflect this.