fix: use isError instead of instanceof due to realms

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lib/node_modules/@stdlib/utils/jsonify-keys/examples/index.js

@@ -18,6 +18,7 @@
 
 'use strict';
 
+var isError = require( '@stdlib/assert/is-error' );
 var jsonifyKeys = require( './../lib' );
 
 var out = jsonifyKeys( '{ beep: "boop" }' );
@@ -37,5 +38,5 @@ console.log( out );
 // => null
 
 out = jsonifyKeys( '{ beep: "boop }' );
-console.log( out instanceof Error );
+console.log( isError( out ) );
 // => true