Updated doc with correct type.

src/doc/tutorial.md

@@ -944,7 +944,7 @@ struct Foo { x: int, y: Box<int> }
     // `a` is the owner of the struct, and thus the owner of the struct's fields
     let a = Foo { x: 5, y: box 10 };
 }
-// when `a` goes out of scope, the destructor for the `~int` in the struct's
+// when `a` goes out of scope, the destructor for the `Box<int>` in the struct's
 // field is called
 
 // `b` is mutable, and the mutability is inherited by the objects it owns