[SPARK-] [MLLIB] minor fix on tokenizer doc

A trivial fix for the comments of RegexTokenizer.

Maybe this is too small, yet I just noticed it and think it can be quite misleading. I can create a jira if necessary.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #7791 from hhbyyh/docFix and squashes the following commits:

cdf2542 [Yuhao Yang] minor fix on tokenizer doc