doc: change write() after end() streams example

Currently there's an example using http.ServerResponse stream, which
has a known bug and will not throw an error while writing after end().
Changed to a writable stream from fs which behaves as expected.

Fixes https://github.com/joyent/node/issues/8814.

PR-URL: https://github.com/iojs/io.js/pull/155
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brendan Ashworth <squirrelslikeacorns@gmail.com>