docs(Empty): update demo button height (#6399)

src/empty/README.md

@@ -58,6 +58,7 @@ Use the image prop to display different placeholder images
 <style>
   .bottom-button {
     width: 160px;
+    height: 40px;
   }
 </style>
 ```

src/empty/README.zh-CN.md

@@ -67,6 +67,7 @@ Empty 组件内置了多种占位图片类型，可以在不同业务场景下
 <style>
   .bottom-button {
     width: 160px;
+    height: 40px;
   }
 </style>
 ```

src/empty/demo/index.vue

@@ -82,6 +82,7 @@ export default {
 
   .bottom-button {
     width: 160px;
+    height: 40px;
   }
 }
 </style>