docs: fixed download_blob_to_file example (#704)

* docs: fixed download_blob_to_file example

* Update google/cloud/storage/client.py

Co-authored-by: cojenco <cathyo@google.com>

Co-authored-by: cojenco <cathyo@google.com>

google/cloud/storage/client.py

@@ -1089,7 +1089,7 @@ def download_blob_to_file(
             >>> from google.cloud import storage
             >>> client = storage.Client()
 
-            >>> with open('file-to-download-to', 'w') as file_obj:
+            >>> with open('file-to-download-to', 'wb') as file_obj:
             >>>     client.download_blob_to_file(
             >>>         'gs://bucket_name/path/to/blob', file_obj)