Fix render_flat check file exists

Fix render_flat check file exists to use Bucket and Key keywords as
part of #111.

Author: sampottinger

snapshot/render_flat.py

@@ -379,7 +379,7 @@ def check_file_exists(full_loc: str) -> bool:
             True if the file is found and false otherwise.
         """
         try:
-            s3_client.head_object(bucket=bucket, key=full_loc)
+            s3_client.head_object(Bucket=bucket, Key=full_loc)
             return True
         except botocore.exceptions.ClientError as e:
             error_code = e.response['Error']['Code']