Use dbg! instead of println! in Day 2. (#2657)

Part of #2478 to clean up code blocks when all that is needed is a
trivial debug print statement.

As mentioned in previous related PRs, in some places I've opted to
retain the use of println! because dbg! makes it less readable.

Co-authored-by: Eric Githinji <egithinji@google.com>

src/pattern-matching/let-control-flow/while-let.md

@@ -4,13 +4,11 @@ Like with `if let`, there is a
 [`while let`](https://doc.rust-lang.org/reference/expressions/loop-expr.html#predicate-pattern-loops)
 variant which repeatedly tests a value against a pattern:
 
-<!-- mdbook-xgettext: skip -->
-
 ```rust,editable
 fn main() {
     let mut name = String::from("Comprehensive Rust 🦀");
     while let Some(c) = name.pop() {
-        println!("character: {c}");
+        dbg!(c);
     }
     // (There are more efficient ways to reverse a string!)
 }