[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 1014668f)
Signed-off-by: Reynold Xin <rxin@apache.org>

docs/configuration.md

@@ -23,7 +23,7 @@ cluster (e.g. master URL and application name), as well as arbitrary key-value p
 `set()` method. For example, we could initialize an application as follows:
 
 {% highlight scala %}
-val conf = new SparkConf
+val conf = new SparkConf()
              .setMaster("local")
              .setAppName("CountingSheep")
              .set("spark.executor.memory", "1g")