chore: fix unvalid wx:key (#2398)

example/pages/dashboard/index.wxml

@@ -8,7 +8,7 @@
   <van-collapse
     wx:for="{{ list }}"
     wx:for-item="group"
-    wx:key="group.title"
+    wx:key="title"
     value="{{ activeNames }}"
     border="{{ false }}"
     bind:change="onChangeCollapse"
@@ -29,7 +29,7 @@
       />
       <van-cell
         wx:for="{{ group.list }}"
-        wx:key="item.title"
+        wx:key="title"
         is-link
         url="/pages{{ item.path }}/index"
         data-url="/pages{{ item.path }}/index"

packages/picker/index.wxml

@@ -38,7 +38,7 @@
     <picker-column
       class="van-picker__column"
       wx:for="{{ isSimple(columns) ? [columns] : columns }}"
-      wx:key="{{ index }}"
+      wx:key="index"
       data-index="{{ index }}"
       custom-class="column-class"
       value-key="{{ valueKey }}"

packages/uploader/index.wxml

@@ -8,7 +8,7 @@
     <block wx:if="{{ previewImage }}">
       <view
         wx:for="{{ lists  }}"
-        wx:key="{{ index }}"
+        wx:key="index"
         class="van-uploader__preview"
       >
         <image