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/tuples-and-arrays/exercise.rs

@@ -33,9 +33,9 @@ fn main() {
         [301, 302, 303],
     ];
 
-    println!("matrix: {:#?}", matrix);
+    dbg!(matrix);
     let transposed = transpose(matrix);
-    println!("transposed: {:#?}", transposed);
+    dbg!(transposed);
 }
 // ANCHOR_END: main
 // ANCHOR_END: solution