docs(worker): correct typings on event failed (#2818)

docs/gitbook/guide/workers/README.md

@@ -71,7 +71,7 @@ worker.on('progress', (job: Job, progress: number | object) => {
 Finally, when the process fails with an exception it is possible to listen for the `failed` event too:
 
 ```typescript
-worker.on('failed', (job: Job, error: Error) => {
+worker.on('failed', (job: Job | undefined, error: Error, prev: string) => {
   // Do something with the return value.
 });
 ```