chore: fix JavaScript lint error in fs/read-dir examples

Replace `out instanceof Error` with `isError( out )` using
`@stdlib/assert/is-error` to comply with the `stdlib/doctest`
linting rule. This matches the pattern used in similar packages
like `@stdlib/fs/read-file`.

Resolves #8483

Co-Authored-By: sumithraju <noreply@github.com>

Author: Sumithraju

lib/node_modules/@stdlib/fs/read-dir/examples/index.js

@@ -18,20 +18,21 @@
 
 'use strict';
 
+var isError = require( '@stdlib/assert/is-error' );
 var readDir = require( './../lib' );
 
 /* Sync */
 
 var out = readDir.sync( __dirname );
 // returns <Array>
 
-console.log( out instanceof Error );
+console.log( isError( out ) );
 // => false
 
 out = readDir.sync( 'beepboop' );
 // returns <Error>
 
-console.log( out instanceof Error );
+console.log( isError( out ) );
 // => true
 
 /* Async */