docs: use writeable streamin example for 'download_blob_to_file' (#676)
docs: use writeable streamin example for 'download_blob_to_file' (#676)

Co-authored-by: Tres Seaver <0c654242ef00c3b4e79f8f8068911d86fe01854e@palladion.com>
googleapis/python-storage,googleapis/python-storage