[SPARK-21508][DOC] Fix example code provided in Spark Streaming Documentation

## What changes were proposed in this pull request?

JIRA ticket : [SPARK-21508](https://issues.apache.org/jira/projects/SPARK/issues/SPARK-21508)

correcting a mistake in example code provided in Spark Streaming Custom Receivers Documentation
The example code provided in the documentation on 'Spark Streaming Custom Receivers' has an error.
doc link : https://spark.apache.org/docs/latest/streaming-custom-receivers.html

```

// Assuming ssc is the StreamingContext
val customReceiverStream = ssc.receiverStream(new CustomReceiver(host, port))
val words = lines.flatMap(_.split(" "))
...
```

instead of `lines.flatMap(_.split(" "))`
it should be `customReceiverStream.flatMap(_.split(" "))`

## How was this patch tested?
this documentation change is tested manually by jekyll build , running below commands
```
jekyll build
jekyll serve --watch
```
screen-shots provided below
![screenshot1](https://user-images.githubusercontent.com/8828470/28744636-a6de1ac6-7482-11e7-843b-ff84b5855ec0.png)
![screenshot2](https://user-images.githubusercontent.com/8828470/28744637-a6def496-7482-11e7-9512-7f4bbe027c6a.png)

Author: Remis Haroon <[email protected]>

Closes #18770 from remisharoon/master.

Author: Remis Haroon

docs/streaming-custom-receivers.md

@@ -175,7 +175,7 @@ an input DStream using data received by the instance of custom receiver, as show
 {% highlight scala %}
 // Assuming ssc is the StreamingContext
 val customReceiverStream = ssc.receiverStream(new CustomReceiver(host, port))
-val words = lines.flatMap(_.split(" "))
+val words = customReceiverStream.flatMap(_.split(" "))
 ...
 {% endhighlight %}
 
@@ -187,7 +187,7 @@ The full source code is in the example [CustomReceiver.scala]({{site.SPARK_GITHU
 {% highlight java %}
 // Assuming ssc is the JavaStreamingContext
 JavaDStream<String> customReceiverStream = ssc.receiverStream(new JavaCustomReceiver(host, port));
-JavaDStream<String> words = lines.flatMap(s -> ...);
+JavaDStream<String> words = customReceiverStream.flatMap(s -> ...);
 ...
 {% endhighlight %}