(nitpick) Chapter 21.1: Clickable link to 127.0.0.1

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • 21.1
  • 127.0.0.1
• I have checked the latest main branch to see if this has already been fixed, in this file:
  • src/ch21-01-single-threaded.md

URL to the section(s) of the book with this problem: https://doc.rust-lang.org/book/ch21-01-single-threaded.html#listening-to-the-tcp-connection

Description of the problem:

Let’s try running this code! Invoke cargo run in the terminal and then load 127.0.0.1:7878 in a web browser.

For the online version of the book, it would be handy if that IP address was a clickable link.

Suggested fix:
Make the address 127.0.0.1:7878 clickable in that paragraph in the online version of the book, but make sure not to introduce ugly changes to the print version. Ideally, the link should open in a separate tab (target="_blank").

Start by reading src/ch21-01-single-threaded.md at the “Listening to the TCP Connection” section, then inspect how the book renders links in its online and print versions. Make 127.0.0.1:7878 clickable online without introducing an unwanted print-version change, and verify the rendered result in both formats.