[Doc] use relative path for links (#4093)

src/button/README.md

@@ -77,7 +77,7 @@ Vue.use(Button);
 ### Route
 
 ```html
-<van-button type="primary" url="//youzan.github.io/vant/mobile.html">URL</van-button>
+<van-button type="primary" url="/vant/mobile.html">URL</van-button>
 <van-button type="primary" to="index">Vue Router</van-button>
 ```
 

src/button/README.zh-CN.md

@@ -95,7 +95,7 @@ Vue.use(Button);
 可以通过`url`属性进行 URL 跳转，或通过`to`属性进行路由跳转
 
 ```html
-<van-button type="primary" url="//youzan.github.io/vant/mobile.html">URL 跳转</van-button>
+<van-button type="primary" url="/vant/mobile.html">URL 跳转</van-button>
 <van-button type="primary" to="index">路由跳转</van-button>
 ```
 

src/button/demo/index.vue

@@ -130,7 +130,7 @@
       <van-button
         :text="$t('urlRoute')"
         type="primary"
-        url="//youzan.github.io/vant/mobile.html"
+        url="/vant/mobile.html"
       />
       <van-button
         :text="$t('vueRoute')"