`whepfrom` rtsp server mode args `--command` up problem

The ffplay don't working

```bash
cargo run --bin=whepfrom -- -w http://localhost:7777/whep/test-rtsp -o rtsp-listen://0.0.0.0:8551 --command \
    "ffplay rtsp://localhost:8551/test-rtsp"
```

You must:

```bash
cargo run --bin=whepfrom -- -w http://localhost:7777/whep/test-rtsp -o rtsp-listen://0.0.0.0:8551
```

And

```bash
ffplay rtsp://localhost:8551/test-rtsp
```