Bug fix for preprocess.sh in quick start.

ISSUE=4611111

git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1469 1ad973e4-5ce8-4261-8a94-b56d1f490c56

demo/quick_start/preprocess.sh

-#!/bin/sh
+#!/bin/bash
 # Copyright (c) 2016 Baidu, Inc. All Rights Reserved
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -33,7 +33,7 @@ test_num=$((min_len/10))
 if [ $test_num -gt 12500 ];then
  test_num=12500
 fi
-train_num=((min_len-test_num))
+train_num=$((min_len-test_num))
 
 head -n$train_num pos.shuffed >train.pos
 head -n$train_num neg.shuffed >train.neg