[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.