Already in the previous edition !
page 381, second code, second of the four puts : animal is a dog, not a cat
puts "It defines methods #{singleton.instance_methods - 'cat'.methods}"
Even if it doesn’t change the result, it should be one of :
puts "It defines methods #{singleton.instance_methods - 'dog'.methods}"
puts %Q(It defines methods #{singleton.instance_methods - "dog".methods})