fix doc typo

Thanks to Mike Sperber for the report.

pkgs/racket-doc/scribblings/reference/custom-ports.scrbl

@@ -1068,7 +1068,7 @@ accum-list
            ;; Cheap strategy: block until the list is unlocked,
            ;;   then return 0, so we get called again
            (wrap-evt
-            lock-peek
+            lock-peek-evt
             (lambda (x) 0))))
      void)))
 (display "hello" accumulator)