update backend_overview.py with provider changes (#2828)
update backend_overview.py with provider changes (#2828)

Replace usage of IBMQ as a provider of backends in backend_overview.py. Without this change, backend_overview throws a deprecation warning.

Fixes #2827
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py