Update JavaCustomReceiver.java

Changed the usage string to correctly reflect the file name.

Author: Karthik <karthik.gomadam@gmail.com>

Closes #2699 from namelessnerd/patch-1 and squashes the following commits:

8570e33 [Karthik] Update JavaCustomReceiver.java

examples/src/main/java/org/apache/spark/examples/streaming/JavaCustomReceiver.java

@@ -57,7 +57,7 @@ public class JavaCustomReceiver extends Receiver<String> {
 
   public static void main(String[] args) {
     if (args.length < 2) {
-      System.err.println("Usage: JavaNetworkWordCount <hostname> <port>");
+      System.err.println("Usage: JavaCustomReceiver <hostname> <port>");
       System.exit(1);
     }