Added explanation on line 125, Ruby tutorial 4

I had trouble understanding the expected outcome when changing the method from protected to private, as the wording implied a different outcome.  I got stuck working through this part of the tutorial, and after discussing with my coach, I've changed the wording to be clearer for other students, stating which error is expected and why.

Author: ruthmoog

ruby/lesson4/tutorial.md

@@ -122,7 +122,7 @@ color = Color.new("purple", "#8824a4")
 color.update_hexcode("#9427b2")
 ```
 
-This should now throw a `NoMethodError`. Change **protected** to **private** and run the code again.
+This should now throw a `NoMethodError`. Change **protected** to **private** and run the code again. This should still throw a `NoMethodError` since the `update_hexcode` method is being called from outside of the class.  Think about how you could call the private method successfully.  Remember, the private method needs to be called from within the class.
 
 Let's now put into practise what we have learned so far.