Fix item-generation command line in README.md

To facilitate the preparation of items for P800-{X} listening tests, it is possible to generate samples of complex formats (STEREO, SBA, ISMn, OMASA, OSBA) from mono samples. To generate items, run the following command from the root of the repository:

```bash

   python generate_items.py --config path/to/scene_description_config_file.yml

   python3 -m ivas_processing_scripts.generation path/to/scene_description_config_file.yml

 ```

The YAML configuration file (`scene_description_config_file.yml`) defines how individual mono files should be spatially positioned and combined into the target format. For advanced formats like OMASA or OSBA, note that additional SBA items may be required. Refer to the `examples/` folder for template `.yml` files demonstrating the expected structure and usage.

---

# Item generation

The `item_generation_scripts` module may be used to generate audio items for the P.800 listening test according to the scene description. All scenes must be fully described in the `SCENE.yml` file. The module takes monophonic audio

files from the specified input directory as the input and store the audio items in the requested format specification to the specified output directory. The module also generates the associated metadata files in case the target format requires so.

This module may be executed from the command-line with `python -m ivas_processing_scripts.generation item_gen_configs/SCENE.YML`.

# Listening test generation

The `ivas_processing_scripts` module helps to quickly setup listening tests with multiple (pre-)processing and post-processing options.