doc: move numCPUs require to top of file in cluster CJS example · nodejs/node@30310bf

```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) {

}

// Start workers and listen for messages containing notifyRequest

const numCPUs = require('node:os').availableParallelism();

for (let i = 0; i < numCPUs; i++) {

cluster.fork();

}