[SPARK-1826] fix the head notation of package object dsl

Author: wangfei <scnbwf@yeah.net>

Closes #765 from scwf/dslfix and squashes the following commits:

d2d1a9d [wangfei] Update package.scala
66ff53b [wangfei] fix the head notation of package object dsl

(cherry picked from commit 44165fc91a31e6293a79031c89571e139d2c5356)
Signed-off-by: Reynold Xin <rxin@apache.org>