update django docs link

chapter_20_mocking_1.asciidoc

@@ -591,7 +591,7 @@ We'll use Django's "messages framework",
 which is often used to display ephemeral "success" or "warning" messages
 to show the results of an action.
 Have a look at the
-https://docs.djangoproject.com/en/1.11/ref/contrib/messages/[django messages docs]
+https://docs.djangoproject.com/en/5.1/ref/contrib/messages/[django messages docs]
 if you haven't come across it already.
 
 Testing Django messages is a bit contorted--we have to pass `follow=True`