update webserver source summaries

[flatcap]

Update the two source summaries for the webserver chapter.

Ref: https://doc.rust-lang.org/stable/book/ch20-03-graceful-shutdown-and-cleanup.html

• Listing 20-25
• Source summary of src/main.rs
  (src/lib.rs matches the exercises)

The two summaries are out-of-date. They don't use match or BufReader.

After working through the examples to build a webserver, the final code should look like this.

[chriskrycho]

Thanks for this! You were not the only one to notice it—@chris-t-jansen noticed the same and fixed it in #3857, which I happened to spot first and so landed a couple weeks ago. Going to close this accordingly, but the fix is very much appreciated nonetheless.