[DOC][MINOR] Kafka doc: breakup into lines

## Before

![before](https://cloud.githubusercontent.com/assets/15843379/20340231/99b039fe-ac1b-11e6-9ba9-b44582427459.png)

## After

![after](https://cloud.githubusercontent.com/assets/15843379/20340236/9d5796e2-ac1b-11e6-92bb-6da40ba1a383.png)

Author: Liwei Lin <lwlin7@gmail.com>

Closes #15903 from lw-lin/kafka-doc-lines.

docs/structured-streaming-kafka-integration.md

@@ -240,6 +240,7 @@ Kafka's own configurations can be set via `DataStreamReader.option` with `kafka.
 [Kafka consumer config docs](http://kafka.apache.org/documentation.html#newconsumerconfigs).
 
 Note that the following Kafka params cannot be set and the Kafka source will throw an exception:
+
 - **group.id**: Kafka source will create a unique group id for each query automatically.
 - **auto.offset.reset**: Set the source option `startingOffsets` to specify
  where to start instead. Structured Streaming manages which offsets are consumed internally, rather