[plt-scheme] Unexpected error using "static-files-path"

I tried the following example, taken from the
tutorial "Continue: Web Applications in PLT Scheme "
As i start the program I receive the following error:

procedure application: procedure: #<procedure:serve/servlet>; 
does not expect an argument with keyword #:extra-files-path; 
arguments were: #<procedure:start> #:extra-files-path #<path:htdocs>
#:launch-browser? #t

Any suggestion ?
Regards
Maurizio.

 #lang web-server/insta
  (define (start request)
    '(html (head (title "Testing"))
           (link ((rel "stylesheet")
                  (href "/test-static.css")
                  (type "text/css")))
           (body (h1 "Testing")
                 (h2 "This is a header")
                 (p "This is " (span ((class "hot")) "hot") "."))))
  
  (static-files-path "htdocs")