Fixed #23900 -- Added missing imports in code example.

The Preventing header injection example included
classes that are not imported.

Thanks to Collin Anderson and Berker Peksağ for the
reviews.

docs/topics/email.txt

@@ -188,6 +188,7 @@ from the request's POST data, sends that to admin@example.com and redirects to
 "/contact/thanks/" when it's done::
 
     from django.core.mail import send_mail, BadHeaderError
+    from django.http import HttpResponse, HttpResponseRedirect
 
     def send_email(request):
         subject = request.POST.get('subject', '')