docs: prefer shorter demo

src/nav-bar/README.md

@@ -30,12 +30,8 @@ import { Toast } from 'vant';
 
 export default {
   setup() {
-    const onClickLeft = () => {
-      Toast('Back');
-    };
-    const onClickRight = () => {
-      Toast('Button');
-    };
+    const onClickLeft = () => Toast('Back');
+    const onClickRight = () => Toast('Button');
     return {
       onClickLeft,
       onClickRight,

src/nav-bar/README.zh-CN.md

@@ -30,12 +30,8 @@ import { Toast } from 'vant';
 
 export default {
   setup() {
-    const onClickLeft = () => {
-      Toast('返回');
-    };
-    const onClickRight = () => {
-      Toast('按钮');
-    };
+    const onClickLeft = () => Toast('返回');
+    const onClickRight = () => Toast('按钮');
     return {
       onClickLeft,
       onClickRight,

src/nav-bar/demo/index.vue

@@ -35,14 +35,8 @@ const i18n = {
 export default {
   setup() {
     const t = useTranslate(i18n);
-
-    const onClickLeft = () => {
-      Toast(t('back'));
-    };
-
-    const onClickRight = () => {
-      Toast(t('button'));
-    };
+    const onClickLeft = () => Toast(t('back'));
+    const onClickRight = () => Toast(t('button'));
 
     return {
       t,