build fix for observing_test.rb

introduced here 17c990b153f8635874c006a7460ee95817543fc1

activemodel/test/cases/observing_test.rb

@@ -113,7 +113,7 @@ def teardown
 
   test "tracks implicit observable models" do
     instance = FooObserver.new
-    assert_equal [Foo], old_instance.observed_classes
+    assert_equal [Foo], instance.observed_classes
   end
 
   test "tracks explicit observed model class" do