doc: fix URL postMessage example in worker_threads

The example claimed that posting a URL object via MessageChannel would
print an empty object, but since v21.0.0 (commit d920b7c) it
throws a DataCloneError. Update the example and surrounding text to
reflect the current behavior.

Fixes: #60504

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Kit Dallege <xaum.io@gmail.com>

Author: kovan