doc: update checkbox label in backporting guide

Signed-off-by: Darshan Sen <darshan.sen@postman.com>

PR-URL: https://github.com/nodejs/node/pull/39420


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>