[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>

docs/structured-streaming-kafka-integration.md

@@ -90,7 +90,7 @@ ds3.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
 
 # Subscribe to 1 topic
 ds1 = spark
-  .readStream()
+  .readStream
   .format("kafka")
   .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
   .option("subscribe", "topic1")
@@ -108,7 +108,7 @@ ds2.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
 
 # Subscribe to a pattern
 ds3 = spark
-  .readStream()
+  .readStream
   .format("kafka")
   .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
   .option("subscribePattern", "topic.*")