--output has no effect

```
⛵ kind-knative Documents/Talks/wjax-knative-2024/demo
10:54 ✗ kn event send -o yaml --to demo -f message=Hello
📬 Sending event within the cluster Done
3582 INFO Event (ID: 9a07dbd5-5767-4368-a611-34649527b076) have been sent.

⛵ kind-knative Documents/Talks/wjax-knative-2024/demo
10:54 ➜ kn event send -o human --to demo -f message=Hello
📬 Sending event within the cluster Done
4093 INFO Event (ID: a5ba200e-fa68-4a1b-a630-3512c88c31ea) have been sent.

⛵ kind-knative Documents/Talks/wjax-knative-2024/demo
10:55 ➜ kn event send -o json --to demo -f message=Hello
📬 Sending event within the cluster Done
3901 INFO Event (ID: 0ea7a3d4-09d1-42e1-9d02-1a6b95fb0e4b) have been sent.
```

What is `--output` supposed to do ?
(minor nit: It's "Event .... has been sent", not "Event ... have been sent")

Start at the `kn event send` command entry point and reproduce the three commands shown with `-o yaml`, `-o human`, and `-o json`. Check whether the output option is parsed and passed through, then define done as the selected format producing visibly different output and the event message using correct grammar.

Written by the indexing model from the issue text.