[SPARK-1250] Fixed misleading comments in bin/pyspark, bin/spark-class

Fixed a couple of misleading comments in bin/pyspark and bin/spark-class. The comments make it seem like the script is looking for the Scala installation when in fact it is looking for Spark.

Author: Sumedh Mungee <smungee@gmail.com>

Closes #843 from smungee/spark-1250-fix-comments and squashes the following commits:

26870f3 [Sumedh Mungee] [SPARK-1250] Fixed misleading comments in bin/pyspark and bin/spark-class

(cherry picked from commit 6e337380fc47071fc7fb28d744e8209c729fe1e9)
Signed-off-by: Reynold Xin <rxin@apache.org>