Your code is written for a different library.
You have 2 options.. try and find the library it was written for... this comment seem to indicate it was linked from the web page you were on...
//Library for dht11 Temperature and Humidity sensor (Download from Link in article)
Use a different library.. the one you now have included DHT.h
But.. you will need to change your code to match. Try the link above for details.