Clarified example with 'rock-band' fiunction; two typos. 
Clarified example with 'rock-band' fiunction; two typos. 

The article referred to a previous function 'rock-band?', but that doesn't exist. Neither existed 'bands' variable. I've searched for it on the internet, but the original article must have been lost. Hence this change makes the example complete.
clojure/clojure-site