Typos in Spark

Author: Huajian Mao <huajianmao@gmail.com>

Closes #798 from huajianmao/patch-1 and squashes the following commits:

208a454 [Huajian Mao] A typo in Task
1b515af [Huajian Mao] A typo in the message

(cherry picked from commit 94c5139607ec876782e594012a108ebf55fa97db)
Signed-off-by: Reynold Xin <rxin@apache.org>