message: docs: fix design resource link
diff: @@ -22,7 +22,7 @@ Contains color specifications, font specifications, and component design specifi
  <img src="https://img.yzcdn.cn/vant/tab_202009101415.png" style="width: 80%; box-shadow: 0 1px 2px rgba(0,0,0,.2)">
  -<a class="design-download" href="https://github.com/youzan/vant/blob/dev/docs/assets/design.sketch?raw=true">Download</a>
  +<a class="design-download" href="https://github.com/youzan/vant/blob/dev/packages/vant/docs/assets/design.sketch?raw=true">Download</a>
  ### Icons (Sketch)
  