Incorrect link to PEP for Zstandard feature in What's New

# Documentation

Currently, [PEP 768](https://peps.python.org/pep-0768/) is linked in the What's New page (added in #133495) instead of [PEP 784](https://peps.python.org/pep-0784/).


### Linked PRs
* gh-133507