docs(Checkbox): update checkbox use with cell demo @rex-zsd (#947)

#937

example/pages/checkbox/index.wxml

@@ -84,6 +84,7 @@
         wx:for="{{ list }}"
         wx:key="{{ index }}"
         title="复选框{{ item }}"
+        value-class="value-class"
         clickable
         data-name="{{ item }}"
         bind:click="toggle"

example/pages/checkbox/index.wxss

@@ -6,6 +6,10 @@
   margin: 10px 0 0 20px;
 }
 
+.value-class {
+  flex: none !important;
+}
+
 .icon {
   width: 20px;
 }