correct tiny comment error

Author: Chen Chao <crazyjvm@gmail.com>

Closes #928 from CrazyJvm/patch-8 and squashes the following commits:

144328b [Chen Chao] correct tiny comment error

(cherry picked from commit 9ecc40d3aeff0eb113f16df55f4249d8143f37f1)
Signed-off-by: Reynold Xin <rxin@apache.org>