Fixed a typo in an example in the timezone docs.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17908 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/topics/i18n/timezones.txt

@@ -199,7 +199,7 @@ Create a view that can set the current timezone::
 
     def set_timezone(request):
         if request.method == 'POST':
-            request.session[session_key] = pytz.timezone(request.POST['timezone'])
+            request.session['django_timezone'] = pytz.timezone(request.POST['timezone'])
             return redirect('/')
         else:
             return render(request, 'template.html', {'timezones': pytz.common_timezones})