doc: improve worker pool example

In the worker pool example, the 'kWorkerFreedEvent' should be emitted
in case of error as well. After adding new worker in the error handler,
the pending tasks should be notified of an available worker.

PR-URL: #33082
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>

doc/api/async_hooks.md

@@ -820,6 +820,7 @@ class WorkerPool extends EventEmitter {
     });
     this.workers.push(worker);
     this.freeWorkers.push(worker);
+    this.emit(kWorkerFreedEvent);
   }
 
   runTask(task, callback) {