fix(box1): fix comment typo
Doesn't effect the code.
// elements: the value of the current item and the next item. The last item is a value called `Nil`.
//
// Step 1: use a `Box` in the enum definition to make the code compile
-// Step 2: create both empty and non-empty cons lists of by replacing `unimplemented!()`
+// Step 2: create both empty and non-empty cons lists by replacing `unimplemented!()`
//
// Note: the tests should not be changed
//
