Updating functions/billing/main.py - Code sample references (#2388)
Updating functions/billing/main.py - Code sample references (#2388)

* Updating main.py

Minor tweaks so the code samples will work.

* Update main.py

Correcting the error checking to use not in.

GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples