[SPARK-26889][SS][DOCS] Fix timestamp type in Structured Streaming + …

…Kafka Integration Guide

## What changes were proposed in this pull request?

```
$ spark-shell --packages org.apache.spark:spark-sql-kafka-0-10_2.11:3.0.0-SNAPSHOT
...
scala> val df = spark.read.format("kafka").option("kafka.bootstrap.servers", "foo").option("subscribe", "bar").load().printSchema()
root
 |-- key: binary (nullable = true)
 |-- value: binary (nullable = true)
 |-- topic: string (nullable = true)
 |-- partition: integer (nullable = true)
 |-- offset: long (nullable = true)
 |-- timestamp: timestamp (nullable = true)
 |-- timestampType: integer (nullable = true)

df: Unit = ()
```
In the doc timestamp type is `long` and in this PR I've changed it to `timestamp`.

## How was this patch tested?

cd docs/
SKIP_API=1 jekyll build
Manual webpage check.

Closes #23796 from gaborgsomogyi/SPARK-26889.

Authored-by: Gabor Somogyi <gabor.g.somogyi@gmail.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

docs/structured-streaming-kafka-integration.md

@@ -265,7 +265,7 @@ Each row in the source has the following schema:
 </tr>
 <tr>
   <td>timestamp</td>
-  <td>long</td>
+  <td>timestamp</td>
 </tr>
 <tr>
   <td>timestampType</td>