updated java code blocks in spark SQL guide such that ctx will refer to ...

...a JavaSparkContext and sqlCtx will refer to a JavaSQLContext

Author: Yadid Ayzenberg <yadid@media.mit.edu>

Closes #932 from yadid/master and squashes the following commits:

f92fb3a [Yadid Ayzenberg] updated java code blocks in spark SQL guide such that ctx will refer to a JavaSparkContext and sqlCtx will refer to a JavaSQLContext

(cherry picked from commit 366c0c4c30465be1a298a5c750b6dbbf022c2323)
Signed-off-by: Reynold Xin <rxin@apache.org>