docs: Update demo (#19059)

* docs: Update demo

* fix lint

components/select/demo/custom-dropdown-menu.md

@@ -14,7 +14,7 @@ title:
 Customize the dropdown menu via `dropdownRender`.
 
 ```jsx
-import { Select, Divider } from 'antd';
+import { Select, Divider, Input } from 'antd';
 import { Plus } from '@ant-design/icons';
 
 const { Option } = Select;
@@ -24,18 +24,26 @@ let index = 0;
 class App extends React.Component {
   state = {
     items: ['jack', 'lucy'],
+    name: '',
+  };
+
+  onNameChange = event => {
+    this.setState({
+      name: event.target.value,
+    });
   };
 
   addItem = () => {
     console.log('addItem');
-    const { items } = this.state;
+    const { items, name } = this.state;
     this.setState({
-      items: [...items, `New item ${index++}`],
+      items: [...items, name || `New item ${index++}`],
+      name: '',
     });
   };
 
   render() {
-    const { items } = this.state;
+    const { items, name } = this.state;
     return (
       <Select
         style={{ width: 240 }}
@@ -44,13 +52,16 @@ class App extends React.Component {
           <div>
             {menu}
             <Divider style={{ margin: '4px 0' }} />
+            <div style={{ display: 'flex', flexWrap: 'nowrap', padding: 8 }}>
+              <Input style={{ flex: 'auto' }} value={name} onChange={this.onNameChange} />
               <a
-              style={{ padding: '8px', display: 'block', cursor: 'pointer' }}
+                style={{ flex: 'none', padding: '8px', display: 'block', cursor: 'pointer' }}
                 onClick={this.addItem}
               >
                 <Plus /> Add item
               </a>
             </div>
+          </div>
         )}
       >
         {items.map(item => (