Docs: Some docstrings still reference oauth2client credentials

[busunkim96]

Example:

google-cloud-python/error_reporting/google/cloud/error_reporting/_logging.py

Lines 33 to 38 in 8b60456

	:type credentials: :class:`oauth2client.client.OAuth2Credentials` or
	:class:`NoneType`
	:param credentials: The OAuth2 Credentials to use for the connection
	owned by this client. If not passed (and if no
	``_http`` object is passed), falls back to the default
	inferred from the environment.

These should be updated to google.auth.credentials.Credentials.

google-cloud-python/logging/google/cloud/logging_v2/gapic/transports/logging_service_v2_grpc_transport.py

Lines 51 to 55 in e993961

	credentials (google.auth.credentials.Credentials): The
	authorization credentials to attach to requests. These
	credentials identify this application to the service. If none
	are specified, the client will attempt to ascertain the
	credentials from the environment.

• https://github.com/googleapis/google-cloud-python/blob/10cc8a8edd79b1f9a8af89b9629d8849e345cdfd/api_core/docs/auth.rst
• https://github.com/googleapis/google-cloud-python/blob/8b60456c43f73d3989137ccf652f115c065c5901/error_reporting/google/cloud/error_reporting/_logging.py
• https://github.com/googleapis/google-cloud-python/blob/f900a080159f3672d41e1431ebb49ae6bdfbe87b/spanner/google/cloud/spanner_v1/client.py
• https://github.com/googleapis/google-cloud-python/blob/53d692517345979b2b4621872d76c1184c987b92/storage/google/cloud/storage/bucket.py
• https://github.com/googleapis/google-cloud-python/blob/20ebf60cb127b8083d06b78093a8f624cb8f9753/storage/google/cloud/storage/blob.py

See https://github.com/googleapis/google-cloud-python/search?q=oauth2client&unscoped_q=oauth2client