run as module in documentation to avoid failing of absolute import paths

[lfoppiano]

If I run with

(article-dataset-builder) Lucas-M1-MacBook-Pro:article-dataset-builder lfoppiano$ python article_dataset_builder/harvest.py 
Traceback (most recent call last):
  File "/Users/lfoppiano/development/github/article-dataset-builder/article_dataset_builder/harvest.py", line 22, in <module>
    from article_dataset_builder.S3 import S3
ModuleNotFoundError: No module named 'article_dataset_builder'

because of the absolute import. This PR updated the documentation so that it can be run as

python -m article_dataset_builder.harvest