Fix example

[ci skip]

Author: blueyed

docs/helpers.rst

@@ -252,7 +252,7 @@ Example
 ::
 
     def test_max_queries(django_assert_max_num_queries):
-        with django_assert_num_queries(3):
+        with django_assert_max_num_queries(3):
             Item.objects.create('foo')
             Item.objects.create('bar')