doc: correct unsafe URL example in http docs

The previous documentation example for converting `request.url` to an
`URL` object was unsafe, as it could allow a server crash through
malformed URL inputs and potentially enable host header attacks.

This commit revises the example to use string concatenation over the
usage of the `baseUrl` and removes the usage of the `req.headers.host`
as the authority part of the url, mitigating both the crash and security
risks by ensuring the host part of the URL remains controlled and
predictable.

Fixes nodejs#52494

Co-authored-by: @astlouisf
Co-authored-by: @samhh

doc/api/http.md

@@ -2886,24 +2886,23 @@ Accept: text/plain
 To parse the URL into its parts:
 
 ```js
-new URL(request.url, `http://${request.headers.host}`);
+new URL(`http://${process.env.HOST ?? 'localhost'}${request.url}`);
 ```
 
-When `request.url` is `'/status?name=ryan'` and `request.headers.host` is
-`'localhost:3000'`:
+When `request.url` is `'/status?name=ryan'` and `process.env.HOST` is undefined:
 
 ```console
 $ node
-> new URL(request.url, `http://${request.headers.host}`)
+> new URL(`http://${process.env.HOST ?? 'localhost'}${request.url}`);
 URL {
-  href: 'http://localhost:3000/status?name=ryan',
-  origin: 'http://localhost:3000',
+  href: 'http://localhost/status?name=ryan',
+  origin: 'http://localhost',
   protocol: 'http:',
   username: '',
   password: '',
-  host: 'localhost:3000',
+  host: 'localhost',
   hostname: 'localhost',
-  port: '3000',
+  port: '',
   pathname: '/status',
   search: '?name=ryan',
   searchParams: URLSearchParams { 'name' => 'ryan' },