Fix data asset example

Delimeter requires quotes.

Author: mastloui-msft

articles/machine-learning/how-to-create-data-assets.md

@@ -248,7 +248,7 @@ paths:
   - pattern: ./*.txt
 transformations:
   - read_delimited:
-      delimiter: ,
+      delimiter: ','
       encoding: ascii
       header: all_files_same_headers
 ```