Tiny documentation wibble

This commit
   commit 83363c8b
   Author: Simon Peyton Jones <simon.peytonjones@gmail.com>
   Date:   Fri Mar 11 16:55:38 2022 +0000

       Use prepareBinding in tryCastWorkerWrapper

refactored completeNonRecX away, but left a Note referring to it.
This MR fixes that Note.