Improved the timezone middleware example slightly.

This change avoids having the timezone leak from a request to the next.

docs/topics/i18n/timezones.txt

@@ -189,6 +189,8 @@ Add the following middleware to :setting:`MIDDLEWARE_CLASSES`::
             tz = request.session.get('django_timezone')
             if tz:
                 timezone.activate(tz)
+            else:
+                timezone.deactivate()
 
 Create a view that can set the current timezone::