doc: fix typo in http.md

A small document update, all other instances of HTTP header on this
page are written as 'Content-Length'. This also matches casing used
with RFC 2616.

PR-URL: https://github.com/nodejs/node/pull/10975


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>