doc: fix incorrect fs.readFileSync example output

This fixes an incorrect example in the documentation for calling
`fs.readFileSync` on a directory. The example was presumably copied from
the documentation for `fs.readFile`, which has an error argument in its
callback.

PR-URL: #20902
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: not-an-aardvark

doc/api/fs.md

@@ -2377,7 +2377,7 @@ fs.readFileSync('<directory>');
 // => [Error: EISDIR: illegal operation on a directory, read <directory>]
 
 //  FreeBSD
-fs.readFileSync('<directory>'); // => null, <data>
+fs.readFileSync('<directory>'); // => <data>
 ```
 
 ## fs.readlink(path[, options], callback)