Fix simple doc typo in Spark Streaming Custom Receiver

The closing parentheses in the constructor in the first code block example is reversed:

diff --git a/docs/streaming-custom-receivers.md b/docs/streaming-custom-receivers.md
index 4e27d65..3fb540c 100644
— a/docs/streaming-custom-receivers.md
+++ b/docs/streaming-custom-receivers.md
@@ -14,7 +14,7 @@ This starts with implementing [NetworkReceiver](api/streaming/index.html#org.apa
The following is a simple socket text-stream receiver.

• class SocketTextStreamReceiver(host: String, port: Int(
  + class SocketTextStreamReceiver(host: String, port: Int)
  extends NetworkReceiver[String]
  {
  protected lazy val blocksGenerator: BlockGenerator =