fix PIL in image classification

ISSUE=4600255

git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1459 1ad973e4-5ce8-4261-8a94-b56d1f490c56

Author: liaogang

doc/demo/image_classification/image_classification.md

@@ -11,7 +11,17 @@ First, download CIFAR-10 dataset. CIFAR-10 dataset can be downloaded from its of
 <https://www.cs.toronto.edu/~kriz/cifar.html>
 
 We have prepared a script to download and process CIFAR-10 dataset. The script will download CIFAR-10 dataset from the official dataset.
-It will convert it to jpeg images and organize them into a directory with the required structure for the tutorial. Make sure that you have installed the python dependency (PIL). If not, you can install it by `pip install PIL` and if you have installed `pip` package.
+It will convert it to jpeg images and organize them into a directory with the required structure for the tutorial. Make sure that you have installed pillow and its dependents.
+Consider the following commands:
+
+1. install pillow dependents
+
+```bash
+sudo apt-get install libjpeg-dev
+pip install pillow
+```
+
+2. download data and preparation
 
 ```bash
 cd demo/image_classification/data/