doc: Boxes diagram shows enum value inside box.

I found the boxes diagram in the tutorial misleading about how the enum worked.

The current diagram makes it seem that there is a separate Cons struct when there is only one type of struct for the  List type, and Nil is drawn almost as if it's not consuming memory.

I'm aware of the optimization that happens for this enum which takes advantage of the fact that pointer cannot be null but this is an implementation detail and not the only one that applies here.  I can add a note below the diagram mentioning this if you like.