example(captcha): fix demo verify captcha too fast

components/captcha/demo/cases/demo0.vue

@@ -101,6 +101,7 @@ export default {
     },
     submit(val) {
       const max = parseFloat(this.maxlength)
+      setTimeout(() => {
         if (!this.limit || max < 0 || val.length === max) {
           if (val !== '1234') {
             this.$refs.captcha.setError('验证码错误，请重新输入')
@@ -111,6 +112,7 @@ export default {
             })
           }
         }
+      }, 300)
     },
     onSend() {
       console.log('click resend button.')