Adjust run_step3.py to the request rate limit of Semantic Scholar API (100 request per 5 minutes window)

> Dear Ernesto,
> Thanks for your question about our API rate limit. 
> The current rate limit is 100 requests per 5-minute window per IP address. Our API will return 500's if overloaded. If you begin to see this, we request you slow down. 
> Please don't hesitate to let me know if you have any more questions about our API.
> Sincerely,
> Linda Wagner, Semantic Scholar Support