doc: use Buffer.byteLength for Content-Length

As the description in http.md:

> If the body contains higher coded characters then
Buffer.byteLength() should be used to determine the number of
bytes in a given encoding.

PR-URL: #7274
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jackson Tian <shyvo1987@gmail.com>

doc/api/http.md

@@ -809,7 +809,7 @@ Example:
 ```js
 var body = 'hello world';
 response.writeHead(200, {
-  'Content-Length': body.length,
+  'Content-Length': Buffer.byteLength(body),
   'Content-Type': 'text/plain' });
 ```
 
@@ -1119,7 +1119,7 @@ var options = {
   method: 'POST',
   headers: {
     'Content-Type': 'application/x-www-form-urlencoded',
-    'Content-Length': postData.length
+    'Content-Length': Buffer.byteLength(postData)
   }
 };