doc: remove obsolete comment in isError() example

One of the util.isError() examples states that a harmony flag
is required. As of v6.0.0, this is no longer true. This commit
removes the out of date reference.

Refs: #5414
PR-URL: #6486
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/api/util.md

@@ -341,7 +341,6 @@ possible to obtain an incorrect result when the `object` argument manipulates
 `@@toStringTag`.
 
 ```js
-// This example requires the `--harmony-tostring` flag
 const util = require('util');
 const obj = { name: 'Error', message: 'an error occurred' };