doc: move numCPUs require to top of file in cluster CJS example

PR-URL: nodejs#53932
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/cluster.md

@@ -317,6 +317,7 @@ if (cluster.isPrimary) {
 ```cjs
 const cluster = require('node:cluster');
 const http = require('node:http');
+const numCPUs = require('node:os').availableParallelism();
 const process = require('node:process');
 
 if (cluster.isPrimary) {
@@ -335,7 +336,6 @@ if (cluster.isPrimary) {
   }
 
   // Start workers and listen for messages containing notifyRequest
-  const numCPUs = require('node:os').availableParallelism();
   for (let i = 0; i < numCPUs; i++) {
     cluster.fork();
   }