Access Control confusion

I read Ruby U.'s Guide
created by Yukihiro M.

and i have a question about access control.
Access Control section explain method following code.

  • def square(n)*
    |* n * n*
    |* end*

Object class has square method.

But it is not allowed access of square method following document.

We are not allowed to explicitly apply the method to an object:

ruby>* “fish”.square(5)*
ERR: (eval):1: private method `square’ called for “fish”:String

But it is operated when i write code above in ruby 1.8.7

What is the matter ?
wrong document ?