[Doc] use v-slot in demos (#3573)

packages/submit-bar/demo/index.vue

@@ -35,7 +35,7 @@
         @submit="onClickButton"
       >
         <van-checkbox v-model="checked">{{ $t('check') }}</van-checkbox>
-        <span slot="tip">
+        <template v-slot:tip>
           {{ $t('tip2') }}
           <span
             class="edit-address"
@@ -43,7 +43,7 @@
           >
             {{ $t('tip3') }}
           </span>
-        </span>
+        </template>
       </van-submit-bar>
     </demo-block>
   </demo-section>

packages/submit-bar/test/__snapshots__/demo.spec.js.snap

@@ -29,11 +29,11 @@ exports[`renders demo correctly 1`] = `
   </div>
   <div>
     <div class="van-submit-bar">
-      <div class="van-submit-bar__tip"><span>
-        你的收货地址不支持同城送, 
+      <div class="van-submit-bar__tip">
+        你的收货地址不支持同城送,
         <span class="edit-address">
           修改地址
-        </span></span></div>
+        </span></div>
       <div class="van-submit-bar__bar">
         <div role="checkbox" tabindex="0" aria-checked="true" class="van-checkbox">
           <div class="van-checkbox__icon van-checkbox__icon--round van-checkbox__icon--checked"><i class="van-icon van-icon-success">