docs: BackTop - demos - english translation (#3081)

* BackTop - basic demo - english translation

* BackTop - custom demo - english translation

* BackTop - basic demo - english translation - fix

* BackTop - custom demo - english translation - fix

components/back-top/demo/basic.md

@@ -19,7 +19,7 @@ import { BackTop } from 'antd';
 ReactDOM.render(
   <div>
     <BackTop />
-    向下滚动后，见右下角灰色按钮
+    Scroll down to see the bottom right blue button.
   </div>
 , mountNode);
 ````

components/back-top/demo/custom.md

@@ -33,7 +33,7 @@ ReactDOM.render(
     <BackTop style={{ bottom: 100 }}>
       <div style={style}>UP</div>
     </BackTop>
-    向下滚动后，见右下角蓝色按钮
+    Scroll down to see the bottom right blue button.
   </div>
 , mountNode);
 ````