Fix variable name in Task Cookbook tutorial

docs/tutorials/task-cookbook.rst

@@ -42,7 +42,7 @@ The cache key expires after some time in case something unexpected happens
     def import_feed(feed_url):
         # The cache key consists of the task name and the MD5 digest
         # of the feed URL.
-        feed_url_digest = md5(feed_url).hexdigest()
+        feed_url_hexdigest = md5(feed_url).hexdigest()
         lock_id = '{0}-lock-{1}'.format(self.name, feed_url_hexdigest)
 
         # cache.add fails if if the key already exists