doc: declare path on example of

Example is not runnable because `path` is used, but it's not
declared.

PR-URL: nodejs/node#48556
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

graal-nodejs/doc/api/async_hooks.md

@@ -768,6 +768,7 @@ import { executionAsyncId } from 'node:async_hooks';
 import fs from 'node:fs';
 
 console.log(executionAsyncId());  // 1 - bootstrap
+const path = '.';
 fs.open(path, 'r', (err, fd) => {
   console.log(executionAsyncId());  // 6 - open()
 });
@@ -778,6 +779,7 @@ const async_hooks = require('node:async_hooks');
 const fs = require('node:fs');
 
 console.log(async_hooks.executionAsyncId());  // 1 - bootstrap
+const path = '.';
 fs.open(path, 'r', (err, fd) => {
   console.log(async_hooks.executionAsyncId());  // 6 - open()
 });