fix a few stray mentions of django 4

chapter_03_unit_test_first_view.asciidoc

@@ -889,7 +889,7 @@ urlpatterns = [
 
 WARNING: If your _urls.py_ looks different or if it mentions a function called
     `url()` instead of `path()`, it's because you've got the wrong version of
-    Django.  This book is written for Django v4.  Take another look at
+    Django.  This book is written for Django v5.  Take another look at
     the <<pre-requisites>> section and get the right version before you
     go any further.
 

chapter_15_simple_form.asciidoc

@@ -234,7 +234,7 @@ and which fields we want it to use.
 like assigning sensible HTML form input types to different types of field,
 and applying default validation.
 Check out the
-https://docs.djangoproject.com/en/4.2/topics/forms/modelforms/[docs]
+https://docs.djangoproject.com/en/5.1/topics/forms/modelforms/[docs]
 for more info.
 
 We now have some different-looking form HTML:

chapter_23_debugging_prod.asciidoc

@@ -5,7 +5,7 @@
 *******************************************************************************
 🚧 Warning, Chapter update for 3e in progress.
 
-This chapter has only been partially updated to Django 4, Ansible+Docker, etc.
+This chapter has only been partially updated to Django 5, Ansible+Docker, etc.
 
 Following along may be tricky, but I hope to have it in better shape soon!
 *******************************************************************************