Add preceding bang to make first test fail

Seems in-line with the other koans at this point
to require the student remove the bang.

koans/about_nil.rb

@@ -2,7 +2,10 @@ require 'edgecase'
 
 class AboutNil < EdgeCase::Koan
   def test_nil_is_an_object
-    assert nil.is_a?(Object), "Unlike NULL in other languages"
+    #  
+    #  Hint:  '!'s negate the response from what follows.
+    #
+    assert !nil.is_a?(Object), "Unlike NULL in other languages"
   end
 
   def test_you_dont_get_null_pointer_errors_when_calling_methods_on_nil