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>

doc/api/fs.md

@@ -2351,7 +2351,7 @@ changes:
 -->
 
 * `fd` {integer}
-* `buffer` {string|Buffer|Uint8Array}
+* `buffer` {Buffer|Uint8Array}
 * `offset` {integer}
 * `length` {integer}
 * `position` {integer}