[SPARK-18426][STRUCTURED STREAMING] Python Documentation Fix for Structured...

[SPARK-18426][STRUCTURED STREAMING] Python Documentation Fix for Structured Streaming Programming Guide

## What changes were proposed in this pull request?

Update the python section of the Structured Streaming Guide from .builder() to .builder

## How was this patch tested?

Validated documentation and successfully running the test example.

Please review https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark

 before opening a pull request.

'Builder' object is not callable object hence changed .builder() to
.builder

Author: Denny Lee <dennylee@gallifrey.local>

Closes #15872 from dennyglee/master.

(cherry picked from commit b91a51bb)
Signed-off-by: Reynold Xin <rxin@databricks.com>

docs/structured-streaming-programming-guide.md

@@ -58,7 +58,7 @@ from pyspark.sql.functions import explode
 from pyspark.sql.functions import split
 
 spark = SparkSession \
-    .builder() \
+    .builder \
     .appName("StructuredNetworkWordCount") \
     .getOrCreate()
 {% endhighlight %}