Update detect.py [(#2174)](GoogleCloudPlatform/python-docs-samples#2174)

1) I got argument parse error when bucket_name=bucket_name is given
2)blob_list[0] gave me folder name

samples/snippets/detect/detect.py

@@ -704,7 +704,7 @@ def async_detect_document(gcs_source_uri, gcs_destination_uri):
     bucket_name = match.group(1)
     prefix = match.group(2)
 
-    bucket = storage_client.get_bucket(bucket_name=bucket_name)
+    bucket = storage_client.get_bucket(bucket_name)
 
     # List objects with the given prefix.
     blob_list = list(bucket.list_blobs(prefix=prefix))
@@ -715,7 +715,7 @@ def async_detect_document(gcs_source_uri, gcs_destination_uri):
     # Process the first output file from GCS.
     # Since we specified batch_size=2, the first response contains
     # the first two pages of the input file.
-    output = blob_list[0]
+    output = blob_list[1]
 
     json_string = output.download_as_string()
     response = json_format.Parse(