Update link to c++ atomic ordering docs (#264)

I'm not sure if this is actually desirable. For one thing, the link I used isn't
locked to c++20 exactly. But it is the same page the [std lib](https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html) links to
so that may be good for something.

I'm also not sure if other references to `C++11` should be updated or removed.
Basically, I feel like _something_ should change here but I'm not really sure what :-\

src/atomics.md

@@ -13,7 +13,7 @@ received some bugfixes since then.)
 Trying to fully explain the model in this book is fairly hopeless. It's defined
 in terms of madness-inducing causality graphs that require a full book to
 properly understand in a practical way. If you want all the nitty-gritty
-details, you should check out the [C++20 draft specification (Section 31)][C++-model].
+details, you should check out the [C++ specification][C++-model].
 Still, we'll try to cover the basics and some of the problems Rust developers
 face.
 
@@ -256,4 +256,4 @@ relaxed operations can be cheaper on weakly-ordered platforms.
 
 
 [C11-busted]: http://plv.mpi-sws.org/c11comp/popl15.pdf
-[C++-model]: http://eel.is/c++draft/atomics.order
+[C++-model]: https://en.cppreference.com/w/cpp/atomic/memory_order