DEFAULT_MAX_BUFFER_SIZE = 9500000L; README.md says 10,000,000

Noticed DEFAULT_MAX_BUFFER_SIZE = 9500000L in the code:
(https://github.com/googleapis/java-pubsub-group-kafka-connector/blob/main/src/main/java/com/google/pubsub/kafka/sink/CloudPubSubSinkConnector.java#L53)
But README.md says 10,000,000.

I changed it in Pull Request:
[412](https://github.com/googleapis/java-pubsub-group-kafka-connector/pull/412)
