[MLLib] Fix example code variable name misspelling in MLLib Feature Extraction guide

Author: RJ Nowling <rnowling@gmail.com>

Closes #2459 from rnowling/tfidf-fix and squashes the following commits:

b370a91 [RJ Nowling] Fix variable name misspelling in MLLib Feature Extraction guide

docs/mllib-feature-extraction.md

@@ -68,7 +68,7 @@ val sc: SparkContext = ...
 val documents: RDD[Seq[String]] = sc.textFile("...").map(_.split(" ").toSeq)
 
 val hashingTF = new HashingTF()
-val tf: RDD[Vector] = hasingTF.transform(documents)
+val tf: RDD[Vector] = hashingTF.transform(documents)
 {% endhighlight %}
 
 While applying `HashingTF` only needs a single pass to the data, applying `IDF` needs two passes: