doc: Boxes diagram shows enum value inside box.

doc/tutorial.md

@@ -1020,10 +1020,10 @@ being destroyed along with the owner. Since the `list` variable above is
 immutable, the whole list is immutable. The memory allocation itself is the
 box, while the owner holds onto a pointer to it:
 
-      Cons cell        Cons cell        Cons cell
-    +-----------+    +-----+-----+    +-----+-----+
-    |  1  |  ~  | -> |  2  |  ~  | -> |  3  |  ~  | -> Nil
-    +-----------+    +-----+-----+    +-----+-----+
+              List box             List box           List box            List box
+            +--------------+    +--------------+    +--------------+    +--------------+
+    list -> | Cons | 1 | ~ | -> | Cons | 2 | ~ | -> | Cons | 3 | ~ | -> | Nil          |
+            +--------------+    +--------------+    +--------------+    +--------------+
 
 An owned box is a common example of a type with a destructor. The allocated
 memory is cleaned up when the box is destroyed.