[Docs] Correct example of creating a new SparkConf

The example code on the configuration page currently does not compile.

Author: Andrew Or <andrewor14@gmail.com>

Closes #842 from andrewor14/conf-docs and squashes the following commits:

aabff57 [Andrew Or] Correct example of creating a new SparkConf

(cherry picked from commit 1014668f2727863fe46f9c75201ee459d093bf0c)
Signed-off-by: Reynold Xin <rxin@apache.org>