`README.md`: Update working draft revision to N4971

[frederick-vs-ja]

N4971 is the latest C++ working draft, it replaces N4964.

Code:

STL/README.md

Line 61 in a888880

We're implementing the latest C++ Working Draft, currently [N4964][], which will eventually become the next C++

STL/README.md

Line 533 in a888880

[N4964]: https://wg21.link/n4964

Previous issue: #4095.

This issue is intended for a new contributor (especially one new to GitHub) to get started with the simplest possible change.

Please feel free to submit a pull request if there isn't one already linked here - no need to ask for permission! 😸

You can (and should) link your pull request to this issue using GitHub's close/fix/resolve syntax.
(in the PR description not the commit message)