fix documentation example for `protect-out'

The example relied on the current code inspector affecting
syntax object created during `eval', but since the switch
from certificates to taints, the relevant code inspector is
determined by the namespace, and a namespace gets its code
inspector at creation time.

Closes PR 12717

collects/scribblings/reference/syntax.scrbl

@@ -982,7 +982,13 @@ as follows.
    (define weak-inspector (make-inspector (current-code-inspector)))
    (define (weak-eval x)
      (parameterize ([current-code-inspector weak-inspector])
-       (eval x)))
+       (define weak-ns (make-base-namespace))
+       (namespace-attach-module (current-namespace)
+                                ''nest
+                                weak-ns)
+       (parameterize ([current-namespace weak-ns])
+         (namespace-require ''nest)
+         (eval x))))
    (require 'nest)
    (list num-eggs num-chicks)
    (weak-eval 'num-eggs)