SPARK-4971: Fix typo in BlockGenerator comment

Author: CodingCat <zhunansjtu@gmail.com>

Closes #3807 from CodingCat/new_branch and squashes the following commits:

5167f01 [CodingCat] fix typo in the comment

streaming/src/main/scala/org/apache/spark/streaming/receiver/BlockGenerator.scala

@@ -116,7 +116,7 @@ private[streaming] class BlockGenerator(
 
   /**
    * Push a single data item into the buffer. After buffering the data, the
-   * `BlockGeneratorListnere.onAddData` callback will be called. All received data items
+   * `BlockGeneratorListener.onAddData` callback will be called. All received data items
    * will be periodically pushed into BlockManager.
    */
   def addDataWithCallback(data: Any, metadata: Any) = synchronized {