[SPARK-26889][SS][DOCS] Fix timestamp type in Structured Streaming + Kafka Integration Guide

[gaborgsomogyi]

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.

[gaborgsomogyi]

cc @vanzin

[SparkQA]

Test build #102386 has finished for PR 23796 at commit f04b63d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HeartSaVioR]

LGTM

[HyukjinKwon]

Merged to master.