update pic link

doc/ABTEST_IN_PADDLE_SERVING.md

@@ -2,7 +2,7 @@
 
 This document will use an example of text classification task based on IMDB dataset to show how to build a A/B Test framework using Paddle Serving. The structure relationship between the client and servers in the example is shown in the figure below.
 
-<img src="/abtest.png" style="zoom:33%;" />
+<img src="abtest.png" style="zoom:33%;" />
 
 Note that:  A/B Test is only applicable to RPC mode, not web mode.
 

doc/ABTEST_IN_PADDLE_SERVING_CN.md

@@ -2,7 +2,7 @@
 
 该文档将会用一个基于IMDB数据集的文本分类任务的例子，介绍如何使用Paddle Serving搭建A/B Test框架，例中的Client端、Server端结构如下图所示。
 
-<img src="/abtest.png" style="zoom:33%;" />
+<img src="abtest.png" style="zoom:33%;" />
 
 需要注意的是：A/B Test只适用于RPC模式，而不适用于WEB模式。