doc: fix incorrect argument type in fs.readSync

The `buffer` argument must be either Buffer or Uint8Array, string
values are not supported.

PR-URL: https://github.com/nodejs/node/pull/18022


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>