doc: fix http2 example with rstWithCancel

Replace the non-existent method rstStreamWithCancel with rstWithCancel

PR-URL: https://github.com/nodejs/node/pull/16365


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>