Amend Confluent Avro converter installation documentation

DBZ image version: debezium/connect:2.0.0.Final

For DBZ-4952, the Confluent Avro converter was removed from the base image in this commit. The above ticket described how documentation should be added on how to install the Confluent Avro converter. However, this was not done, documentation on how to do this is missing.

Using the current documentation as a guide will give you the error:

org.apache.kafka.common.config.ConfigException: Invalid value io.confluent.connect.avro.AvroConverter for configuration key.converter: Class io.confluent.connect.avro.AvroConverter could not be found.

To resolve this, the documentation should be amended.