Fix README examples (causing low performance)

[kirqz23]

Hi,

There is an issue in README examples:
https://github.com/heroku/salesforce-bulk/blame/main/README.rst#L66
https://github.com/heroku/salesforce-bulk/blame/main/README.rst#L75

IteratorBytesIO is not needed cause the function already returns it.

salesforce-bulk/salesforce_bulk/salesforce_bulk.py

Line 503 in 90fbdab

return util.IteratorBytesIO(iter)

What I've noticed is that when you run result = json.load(IteratorBytesIO(result)) instead of result = json.load(result) my script is running 35 min vs 2 min. It can cause problems for other users as well in opened issues.