[web2py] small typo in Book: Caching with Redis

[Ralo Tannahill]

http://www.web2py.com/books/default/chapter/29/13#Caching-with-Redis

"_cache" missing in the module name:

from gluon.contrib.redis*_cache* import RedisCache
cache.redis = RedisCache('localhost:6379',db=None, debug=True)

Best regards

--