[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

(cherry picked from commit fec921552ffccc36937214406b3e4a050eb0d8e0)
Signed-off-by: Xiangrui Meng <meng@databricks.com>