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>

doc/api/http.md

@@ -1429,7 +1429,7 @@ There are a few special headers that should be noted.
 * Sending a 'Connection: keep-alive' will notify Node.js that the connection to
   the server should be persisted until the next request.
 
-* Sending a 'Content-length' header will disable the default chunked encoding.
+* Sending a 'Content-Length' header will disable the default chunked encoding.
 
 * Sending an 'Expect' header will immediately send the request headers.
   Usually, when sending 'Expect: 100-continue', you should both set a timeout