Use dbg! instead of println! in deep dive sessions. (#2676)

Part of #2478 to clean up code blocks when all that is needed is a
trivial debug print statement. The deep dive sessions didn't have that
many occurrences of trivial println! statements.

I believe this can close #2478.

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

src/bare-metal/useful-crates/spin.md

@@ -15,9 +15,9 @@ use spin::mutex::SpinMutex;
 static COUNTER: SpinMutex<u32> = SpinMutex::new(0);
 
 fn main() {
-    println!("count: {}", COUNTER.lock());
+    dbg!(COUNTER.lock());
     *COUNTER.lock() += 2;
-    println!("count: {}", COUNTER.lock());
+    dbg!(COUNTER.lock());
 }
 ```
 

src/concurrency/threads/scoped.md

@@ -12,7 +12,7 @@ use std::thread;
 fn foo() {
     let s = String::from("Hello");
     thread::spawn(|| {
-        println!("Length: {}", s.len());
+        dbg!(s.len());
     });
 }
 
@@ -30,7 +30,7 @@ fn foo() {
     let s = String::from("Hello");
     thread::scope(|scope| {
         scope.spawn(|| {
-            println!("Length: {}", s.len());
+            dbg!(s.len());
         });
     });
 }