async_hooks,doc: cache process.stdout.fd outside init

doc/api/async_hooks.md

@@ -271,16 +271,18 @@ created, while `triggerAsyncId` shows *why* a resource was created.
 The following is a simple demonstration of `triggerAsyncId`:
 
 ```js
+const { fd } = process.stdout;
+
 async_hooks.createHook({
   init(asyncId, type, triggerAsyncId) {
     const eid = async_hooks.executionAsyncId();
     fs.writeSync(
-      process.stdout.fd,
+      fd,
       `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
   }
 }).enable();
 
-require('net').createServer((conn) => {}).listen(8080);
+net.createServer((conn) => {}).listen(8080);
 ```
 
 Output when hitting the server with `nc localhost 8080`:
@@ -322,33 +324,35 @@ callback to `listen()` will look like. The output formatting is slightly more
 elaborate to make calling context easier to see.
 
 ```js
+const { fd } = process.stdout;
+
 let indent = 0;
 async_hooks.createHook({
   init(asyncId, type, triggerAsyncId) {
     const eid = async_hooks.executionAsyncId();
     const indentStr = ' '.repeat(indent);
     fs.writeSync(
-      process.stdout.fd,
+      fd,
       `${indentStr}${type}(${asyncId}):` +
       ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
   },
   before(asyncId) {
     const indentStr = ' '.repeat(indent);
-    fs.writeSync(process.stdout.fd, `${indentStr}before:  ${asyncId}\n`);
+    fs.writeSync(fd, `${indentStr}before:  ${asyncId}\n`);
     indent += 2;
   },
   after(asyncId) {
     indent -= 2;
     const indentStr = ' '.repeat(indent);
-    fs.writeSync(process.stdout.fd, `${indentStr}after:  ${asyncId}\n`);
+    fs.writeSync(fd, `${indentStr}after:  ${asyncId}\n`);
   },
   destroy(asyncId) {
     const indentStr = ' '.repeat(indent);
-    fs.writeSync(process.stdout.fd, `${indentStr}destroy:  ${asyncId}\n`);
+    fs.writeSync(fd, `${indentStr}destroy:  ${asyncId}\n`);
   },
 }).enable();
 
-require('net').createServer(() => {}).listen(8080, () => {
+net.createServer(() => {}).listen(8080, () => {
   // Let's wait 10ms before logging the server started.
   setTimeout(() => {
     console.log('>>>', async_hooks.executionAsyncId());