Update Format option description in README

README.md

@@ -31,7 +31,7 @@ $ bash make.sh build_linux
 | ----------------|------------------------------------------------|----------------|
 | Project         | Google Cloud project ID | NONE(required) |
 | Topic           | Google Cloud Pub/Sub topic name | NONE(required) |
-| Format          | The format to encode the message. Supported formats are json and map(only value) | map (optional) |
+| Format          | The type of message to be sent to pubsub. Currently, only `json` is supported. | NONE(optional) |
 | Attributes      | JSON string specifying message attributes | NONE(optional) |
 | Debug           | Print debug log | false(optional) |
 | Timeout         | The maximum time that the client will attempt to publish a bundle of messages. (millsecond) | 60000 (optional)|