Fix dead link to resnet demo in Chinese doc

ISSUE=4609079


git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1460 1ad973e4-5ce8-4261-8a94-b56d1f490c56

Author: liuyuan04

doc_cn/demo/index.rst

@@ -21,5 +21,5 @@
 
 常用模型
 ''''''''
-* `ImageNet: ResNet <../../doc/demo/imagenet_model/index.html>`_
+* `ImageNet: ResNet <../../doc/demo/imagenet_model/resnet_model.html>`_
 * `Embedding: Chinese Word <../../doc/demo/embedding_model/index.html>`_