fixed: 修复half-screen-dialog组件中，描述内容slot判断条件有误的问题（根据title属性进行判断），修正为desc属性

Author: howietang

src/half-screen-dialog/half-screen-dialog.wxml

@@ -19,7 +19,7 @@
  </view>
  </view>
  <view class="weui-half-screen-dialog__bd">
- <block wx:if="{{title}}">
+ <block wx:if="{{desc}}">
  <view class="weui-half-screen-dialog__desc">{{desc}}</view>
  <view class="weui-half-screen-dialog__tips">{{tips}}</view>
  </block>