README: Modify basic usage examples

[anthonydb]

(Please feel free to delete this template if it doesn't apply - only use it if it's helpful)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Thanks for this great library!

This PR updates the basic usage example in the README to fix a couple of issues:

• Adds import for the requests library, which is needed for the final block
• Retrieves contributions from 'Schedule A' instead of 'Schedule B'
• Update reference to the contribution_amount dictionary key

Note that I used a filing that has both Schedule A and Schedule B items.

• What is the current behavior? (You can also link to an open issue here)

As presented, code produces import error and key not found error.

Thanks again! This library is invaluable!