Fixed #23847 -- Improved the email_check example in the auth documentation.

docs/topics/auth/default.txt

@@ -500,7 +500,7 @@ The simple way is to run your test on :attr:`request.user
 checks to make sure the user has an email in the desired domain::
 
     def my_view(request):
-        if not '@example.com' in request.user.email:
+        if not request.user.email.endswith('@example.com'):
             return HttpResponse("You can't vote in this poll.")
         # ...
 
@@ -511,7 +511,7 @@ checks to make sure the user has an email in the desired domain::
         from django.contrib.auth.decorators import user_passes_test
 
         def email_check(user):
-            return '@example.com' in user.email
+            return user.email.endswith('@example.com')
 
         @user_passes_test(email_check)
         def my_view(request):