docs(diamond_creation): improve code example clarity

subjects/diamond_creation/README.md

@@ -33,6 +33,9 @@ use diamond_creation::*;
 fn main() {
     println!("{:?}", get_diamond('A'));
     println!("{:?}", get_diamond('C'));
+    for line in get_diamond('C') {
+        println!("{}", line);
+    }
 }
 ```
 
@@ -42,6 +45,11 @@ And its output:
 $ cargo run
 ["A"]
 ["  A  ", " B B ", "C   C", " B B ", "  A  "]
+  A  
+ B B 
+C   C
+ B B 
+  A  
 $
 ```