docs: python log example should set levels at root logger, so that no logs are filter out

[tczhao]

Python root logger has default level warning. With the current example, info and debug log will get filtered out

we should have additional line such as logging.getLogger().setLevel(logging.NOTSET), so that all logs are processed https://github.com/open-telemetry/opentelemetry-python/blob/af83ef151fae90a2bd96224add771e9bb20c33df/docs/examples/logs/example.py#L37