docs(DropdownMenu): add demo of active-color (#4331)

src/dropdown-menu/demo/index.vue

@@ -42,6 +42,19 @@
       </van-dropdown-menu>
     </demo-block>
 
+    <demo-block :title="$t('customActiveColor')">
+      <van-dropdown-menu active-color="#f44">
+        <van-dropdown-item
+          v-model="value1"
+          :options="option1"
+        />
+        <van-dropdown-item
+          v-model="value2"
+          :options="option2"
+        />
+      </van-dropdown-menu>
+    </demo-block>
+
     <demo-block :title="$t('expandDirection')">
       <van-dropdown-menu direction="up">
         <van-dropdown-item
@@ -76,12 +89,13 @@
 export default {
   i18n: {
     'zh-CN': {
-      customContent: '自定义菜单内容',
       disableMenu: '禁用菜单',
       switchTitle1: '包邮',
       switchTitle2: '团购',
       itemTitle: '筛选',
       expandDirection: '向上展开',
+      customContent: '自定义菜单内容',
+      customActiveColor: '自定义选中态颜色',
       option1: [
         { text: '全部商品', value: 0 },
         { text: '新款商品', value: 1 },
@@ -94,12 +108,13 @@ export default {
       ]
     },
     'en-US': {
-      customContent: 'Custom Content',
       disableMenu: 'Disable Menu',
       switchTitle1: 'Title',
       switchTitle2: 'Title',
       itemTitle: 'Title',
       expandDirection: 'Expand Direction',
+      customContent: 'Custom Content',
+      customActiveColor: 'Custom Active Color',
       option1: [
         { text: 'Option1', value: 0 },
         { text: 'Option2', value: 1 },