revert: 恢复误删的placeholder功能

src/App.vue

@@ -56,7 +56,7 @@
             <ul v-if="0 < rules.length" class="list">
                 <li
                     @mouseenter="mouseenterHandler(index)"
-                    v-for="({title, rule, events}, index) in rules"
+                    v-for="({title, rule, events,example}, index) in rules"
                     :key="title"
                     class="row"
                 >
@@ -76,6 +76,7 @@
                             <input
                                 ref="input"
                                 v-model="list[index].value"
+                                :placeholder="example"
                                 @blur="check(index, 'blur')"
                                 @keyup="check(index, 'keyup')"
                             >