docs: change tab's 'Before Change' into a real async function (#12693)

packages/vant/src/tab/README.md

@@ -219,7 +219,7 @@ export default {
 
       // async
       return new Promise((resolve) => {
-        resolve(index !== 3);
+        setTimeout(() => resolve(index !== 3), 1000);
       });
     };
 

packages/vant/src/tab/README.zh-CN.md

@@ -231,7 +231,7 @@ export default {
       // 返回 Promise 来执行异步逻辑
       return new Promise((resolve) => {
         // 在 resolve 函数中返回 true 或 false
-        resolve(index !== 3);
+        setTimeout(() => resolve(index !== 3), 1000);
       });
     };
 

packages/vant/src/tab/demo/index.vue

@@ -65,7 +65,7 @@ const beforeChange = (name: number) => {
     return false;
   }
   return new Promise<boolean>((resolve) => {
-    resolve(name !== 3);
+    setTimeout(() => resolve(name !== 3), 1000);
   });
 };
 </script>