HTTP Module: request.reusedSocket documentation sample code error fix

HTTP Module: request.reusedSocket documentation sample code has a error, their is no agent variable declared in the sample code but that agent variable is used in the request object, which is really confusing for the reader, from where does the agent variable is taken.

So, fixed the code by adding the agent variable and taking the http.agent initalization.

Author: koushil-mankali

doc/api/http.md

@@ -1139,6 +1139,7 @@ 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
@@ -1160,6 +1161,7 @@ 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