fix(box1): fix comment typo (rust-lang#426)

Doesn't effect the code.

exercises/standard_library_types/box1.rs

@@ -10,7 +10,7 @@
 // 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
 //