Fix callout number in 3

Author: hjwp

chapter_03_unit_test_first_view.asciidoc

@@ -706,7 +706,7 @@ class HomePageTest(TestCase):
     We use this instead of manually creating a request object
     and calling the view function directly
 
-<2> Django also provides some assertion helpers like `assertContains`
+<3> Django also provides some assertion helpers like `assertContains`
     that save us from having to manually extract and decode response content,
     and have some other nice properties besides, as we'll see.