Use dbg! instead of println! in Day 1 aft session (#2654)

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

In certain slides (8.1, 9.2, 9.3, 10.5) I've opted to retain the use of
println! because dbg! makes it less readable. The
dbg! macro uses pretty-printing by default and this results in a simple
array such as the one in 8.1 being printed vertically instead of a
cleaner one-liner.

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

src/references/dangling.md

@@ -9,15 +9,13 @@ use. One rule is that references can never be `null`, making them safe to use
 without `null` checks. The other rule we'll look at for now is that references
 can't _outlive_ the data they point to.
 
-<!-- mdbook-xgettext: skip -->
-
 ```rust,editable,compile_fail
 fn main() {
     let x_ref = {
         let x = 10;
         &x
     };
-    println!("x: {x_ref}");
+    dbg!(x_ref);
 }
 ```
 

src/references/shared.md

@@ -8,18 +8,16 @@ A reference provides a way to access another value without taking ownership of
 the value, and is also called "borrowing". Shared references are read-only, and
 the referenced data cannot change.
 
-<!-- mdbook-xgettext: skip -->
-
 ```rust,editable
 fn main() {
     let a = 'A';
     let b = 'B';
 
     let mut r: &char = &a;
-    println!("r: {}", *r);
+    dbg!(*r);
 
     r = &b;
-    println!("r: {}", *r);
+    dbg!(*r);
 }
 ```