Update to scribble/example in section "4.1.5 Showing Racket Examples"

[shunlog]

This section still uses interaction-eval from scribble/eval instead of the newer examples with #:hidden from scribble/examples.

I think these are the necessary changes, instead of:

@(require scribble/eval ...)

@interaction-eval[#:eval helper-eval
                   (require "helper.rkt")]

It should be:

@(require scribble/example ...)

@examples[#:hidden #:eval helper-eval
   (require "helper.rkt")]