`README.md`: Update working draft revision to N4950

[frederick-vs-ja]

N4950 is the latest C++ working draft, it replaces N4944.

Code:

STL/README.md

Line 61 in 5404ba9

We're implementing the latest C++ Working Draft, currently [N4944][], which will eventually become the next C++

STL/README.md

Line 535 in 5404ba9

[N4944]: https://wg21.link/n4944

Previous issue: #3590.

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)