minor rewording in the tutorial's `Rc` coverage

doc/tutorial.md

@@ -1320,7 +1320,8 @@ let z = x; // this moves `x` into `z`, rather than creating a new owner
 
 assert_eq!(*z.borrow(), [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
 
-let mut a = Rc::new([10, 9, 8, 7, 6, 5, 4, 3, 2, 1]); // the variable is mutable, but not the box
+// the variable is mutable, but not the contents of the box
+let mut a = Rc::new([10, 9, 8, 7, 6, 5, 4, 3, 2, 1]);
 a = z;
 ~~~