Broken link in Chapter 16.4 in section: "Implementing Send and Sync manually is unsafe"

[polyagonal1]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • link
  • Rustonomicon
  • nomicon
  • chapter 16
  • ch 16
  • ch 16.4
  • chapter 16.4
• I have checked the latest main branch to see if this has already been fixed, in this file:
  • ch16-04-extensible-concurrency-sync-and-send.md

URL to the section(s) of the book with this problem: Chapter 16.4 - Implementing Send and Sync manually is unsafe

Description of the problem: The link at line 71-72 of Chapter 16.4: [“The Rustonomicon”][nomicon] links to a nonexistent URL: https://github.com/cognitive-engineering-lab/rust-book/blob/main/nomicon/index.html.

Suggested fix: Change the link to point to the official Rustonomicon.

This is my first issue, so if there is anything I could do better, do tell me.

[polyagonal1]

I assume this issue can be closed now because the changes in the PR were rolled up to #394.