Docs: Add missing _downcast_ns_timestamp_to_us to ArrowScan docstring #9156

Job	Run time
cibw-dev-env-smoke-test	26s
integration-test	11m 32s
integration-test-gcs	1m 14s
lint-and-unit-test (3.10)	3m 19s
lint-and-unit-test (3.11)	3m 18s
lint-and-unit-test (3.13)	2m 58s
integration-test-adls	2m 9s
lint-and-unit-test (3.12)	3m 9s
lint-and-unit-test (3.14)	2m 43s
integration-test-s3	1m 8s
integration-coverage-report	28s
	32m 24s

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