Extend the mutex singleton example with object substitution

Author: fadeevab

creational/singleton/how-to-create/README.md

@@ -118,5 +118,6 @@ cargo run --bin singleton-mutex
 ### Output
 
 ```
-Called 3 times
+Called 3 times: [1, 1, 1]
+New singleton object: [3, 4, 5]
 ```

creational/singleton/how-to-create/mutex.rs

@@ -21,5 +21,11 @@ fn main() {
     do_a_call();
     do_a_call();
 
-    println!("Called {} times", ARRAY.lock().unwrap().len());
+    let array = ARRAY.lock().unwrap();
+    println!("Called {} times: {:?}", array.len(), array);
+    drop(array);
+
+    *ARRAY.lock().unwrap() = vec![3, 4, 5];
+
+    println!("New singleton object: {:?}", ARRAY.lock().unwrap());
 }