update docs/app-icons.md.

docs/app-icons.md

@@ -117,6 +117,6 @@ iOS系统会缓存应用图标，需要重启手机新图标才能生效
 ```javascript
 ERROR ITMS-90717: "Invalid App Store Icon. The App Store Icon in the asset catalog in 'HBuilder.app' can't be transparent nor contain an alpha channel."
 ```
-**解决方案** 
+**解决方案**  
 生成png图标文件时去掉alpha通道，重新提价云端打包