There is a typo in http://spark.apache.org/docs/latest/structured-streaming-kafka-integration.html#creating-a-kafka-source-stream , python example n1 uses `readStream()` instead of `readStream`

Just removed the parenthesis.

Author: manugarri <manuel.garrido.pena@gmail.com>

Closes #16836 from manugarri/fix_kafka_python_doc.