doc: add missing variable in code snippet

`node:http`' `request.reusedSocket` documentation sample code was
referencing an undeclared `agent` identifier.

PR-URL: #55478
Reviewed-By: Ethan Arrowood <ethan@arrowood.dev>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: koushil-mankali

doc/api/http.md

@@ -1172,6 +1172,7 @@ may run into a 'ECONNRESET' error.
 
 ```mjs
 import http from 'node:http';
+const agent = new http.Agent({ keepAlive: true });
 
 // Server has a 5 seconds keep-alive timeout by default
 http
@@ -1193,6 +1194,7 @@ setInterval(() => {
 
 ```cjs
 const http = require('node:http');
+const agent = new http.Agent({ keepAlive: true });
 
 // Server has a 5 seconds keep-alive timeout by default
 http