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/std-types/hashmap.md

@@ -35,7 +35,7 @@ fn main() {
         *page_count += 1;
     }
 
-    println!("{page_counts:#?}");
+    dbg!(page_counts);
 }
 ```
 

src/std-types/option.md

@@ -13,10 +13,10 @@ returns an `Option<usize>`.
 fn main() {
     let name = "Löwe 老虎 Léopard Gepardi";
     let mut position: Option<usize> = name.find('é');
-    println!("find returned {position:?}");
+    dbg!(position);
     assert_eq!(position.unwrap(), 14);
     position = name.find('Z');
-    println!("find returned {position:?}");
+    dbg!(position);
     assert_eq!(position.expect("Character not found"), 0);
 }
 ```