docs: update tabs extra demo (#26968)

components/tabs/__tests__/__snapshots__/demo.test.js.snap

@@ -1519,8 +1519,7 @@ Array [
         class="ant-tabs-extra-content"
       >
         <button
-          class="ant-btn"
-          style="margin-right:15px"
+          class="ant-btn tabs-extra-demo-button"
           type="button"
         >
           <span>

components/tabs/demo/extra.md

@@ -23,7 +23,7 @@ const CheckboxGroup = Checkbox.Group;
 const operations = <Button>Extra Action</Button>;
 
 const OperationsSlot = {
-  left: <Button style={{ marginRight: 15 }}>Left Extra Action</Button>,
+  left: <Button className="tabs-extra-demo-button">Left Extra Action</Button>,
   right: <Button>Right Extra Action</Button>,
 };
 
@@ -85,3 +85,14 @@ const Demo = () => {
 
 ReactDOM.render(<Demo />, mountNode);
 ```
+
+```css
+.tabs-extra-demo-button {
+  margin-right: 16px;
+}
+
+.ant-row-rtl .tabs-extra-demo-button {
+  margin-right: 0;
+  margin-left: 16px;
+}
+```