Chapter 17.5 JoinAll missing code tags

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • Chapter 17.5
  • JoinAll
• I have checked the latest main branch to see if this has already been fixed, in this file:
  • https://github.com/rust-lang/book/blob/main/src/ch17-05-traits-for-async.md

URL to the section(s) of the book with this problem: https://doc.rust-lang.org/stable/book/ch17-05-traits-for-async.html

Description of the problem: On "The Pin Type and the Unpin Trait", on the 7th paragraph, JoinAll does not have code tags, which may lead to misdirection.

Suggested fix: Add code tags around "JoinAll"