Cannot set offset.flush.interval.ms via docker entrypoint

It's expected that by setting the `$CONNECT_OFFSET_FLUSH_TIMEOUT_MS` environment variable you can change the value of the setting `offset.flush.interval.ms`.

Due to a typo in the `docker-entrypoint.sh` script it's not possible, see the version at 0.9.2.Final:
https://github.com/debezium/docker-images/blob/v0.9.2.Final/connect-base/0.9/docker-entrypoint.sh#L140

To fix it we can rename the variable by renaming the `K` prefix