doc: remove warning from `response.writeHead`

The example referenced as being potentially unsafe specifies
Content-Length correctly.

PR-URL: https://github.com/nodejs/node/pull/32700


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>