Added an Alternative way for printing struct and declaring a string. (#1773)

Knowing to_string method is good for learner to use at some situation. 
Knowing print complete struct object can be useful while debug.

---------

Co-authored-by: Dustin J. Mitchell <djmitche@google.com>

src/types-and-values/strings.md

@@ -53,4 +53,7 @@ later, but this is enough for subsequent slides and exercises to use strings.
   }
   ```
 
+- Using `{:?}` is a convenient way to print array/vector/struct of values for
+  debugging purposes, and it's commonly used in code.
+
 </details>