doc: fix module customization hook examples

When running these examples, `node` fails to return as this
`MessagePort` keeps the event loop active in the main thread unless
it is `unref()`ed.

Fixes: #52846
PR-URL: #53637
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>

doc/api/module.md

@@ -321,6 +321,7 @@ const { port1, port2 } = new MessageChannel();
 port1.on('message', (msg) => {
   console.log(msg);
 });
+port1.unref();
 
 register('./my-hooks.mjs', {
   parentURL: import.meta.url,
@@ -341,6 +342,7 @@ const { port1, port2 } = new MessageChannel();
 port1.on('message', (msg) => {
   console.log(msg);
 });
+port1.unref();
 
 register('./my-hooks.mjs', {
   parentURL: pathToFileURL(__filename),
@@ -431,6 +433,7 @@ const { port1, port2 } = new MessageChannel();
 port1.on('message', (msg) => {
   assert.strictEqual(msg, 'increment: 2');
 });
+port1.unref();
 
 register('./path-to-my-hooks.js', {
   parentURL: import.meta.url,
@@ -453,6 +456,7 @@ const { port1, port2 } = new MessageChannel();
 port1.on('message', (msg) => {
   assert.strictEqual(msg, 'increment: 2');
 });
+port1.unref();
 
 register('./path-to-my-hooks.js', {
   parentURL: pathToFileURL(__filename),