Add a demonstration for the super keyword

restaurant/src/lib.rs

@@ -1,3 +1,5 @@
+fn deliver_order() {}
+
 mod front_of_house {
     pub mod hosting {
         pub fn add_to_waitlist() {}
@@ -14,6 +16,15 @@ mod front_of_house {
     }
 }
 
+mod back_of_house {
+    fn fix_incorrect_order() {
+        cook_order();
+        super::deliver_order();
+    }
+
+    fn cook_order() {}
+}
+
 pub fn eat_at_restaurant() {
     // Absolute path
     crate::front_of_house::hosting::add_to_waitlist();