Getting started example for "from folder" seems to be broken + fix

[YonatanOfek]

Hey,

README.md:163 reads:
python speech_dataset_generator/main.py --input_folder /path/to/folder/of/audios --output_directory /output/directory --range_times 4-10 --enhancers deepfilternet

but yields the following exception:

TypeError: 'NoneType' object is not iterable

from dataset_generator.py:459

Running instead:

python speech_dataset_generator/main.py --input_folder /path/to/folder/of/audios --output_directory /output/directory --range_times 4-10 --enhancers deepfilternet --datasets librispeech

Seems to work fine.

Thanks!