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removes the out of date reference.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: https://github.com/nodejs/node/pull/5414
PR-URL: https://github.com/nodejs/node/pull/6486


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>