fix(demo): fix basic form page style

修复表单基础示例页面，远程下拉的弹出层有时显示不完整的问题。

src/views/demo/form/index.vue

 <template>
-  <PageWrapper title="表单基础示例">
+  <PageWrapper title="表单基础示例" contentFullHeight>
     <CollapseContainer title="基础示例">
       <BasicForm
         autoFocusFirstItem