[MINOR][DOC] Remove parenthesis in readStream() on kafka structured streaming doc

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.

(cherry picked from commit 5a0569ce)
Signed-off-by: Shixiong Zhu <shixiong@databricks.com>