[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
Update docs to use jsonRDD instead of wrong jsonRdd.

Author: Grega Kespret <grega.kespret@gmail.com>

Closes #2479 from gregakespret/patch-1 and squashes the following commits:

dd6b90a [Grega Kespret] Update docs to use jsonRDD instead of wrong jsonRdd.
Merge branch 'master' of https://github.com/apache/spark
Merge branch 'master' into spark-2706