[racket] Cannot reproduce example in "Getting Started with Documentation"

I can't to reproduce the Scribble example  in section 4.1.4 of "Getting 
Started with Documentation".  The procedure 'my-helper' is undefined in 
the html output:
"....
     Examples:
>(my-helper'())
     my-helper: undefined;
cannot reference undefined identifier
     >(my-helper'(cowssuchremarkablecows))
     my-helper: undefined;
cannot reference undefined identifier ..."

I put 'manual.scrbl', 'helper.rkt' in a local collection directory 
'~/.racket/6.1.1/collects/my-lib/' and ran
'raco setup -l my-lib' to produce index.html in my-lib/doc/manual.

I've attached the two files. Can anyone please help?

Cheers - Stuart




-------------- next part --------------
#lang scribble/manual
@(require scribble/eval
          "helper.rkt"
          (for-label racket
                     "helper.rkt"))

@title{My Library}

@defmodule[my-lib/helper]

@defproc[(my-helper [lst list?])
         (listof
          (not/c (one-of/c 'cow)))]{
 Replaces each @racket['cow] in @racket[lst] with
 @racket['aardvark].
 
 @examples[
     (my-helper '())
     (my-helper '(cows such remarkable cows))
   ]}
-------------- next part --------------
#lang racket

(provide my-helper)

(define (my-helper lst)
  (cons "The answer is not:" lst))