update back-top demo

components/back-top/__tests__/__snapshots__/demo.test.js.snap

@@ -16,7 +16,7 @@ exports[`renders ./components/back-top/demo/custom.md correctly 1`] = `
 <div>
   Scroll down to see the bottom-right
   <strong
-    style="color:#57c5f7;"
+    style="color:#1088e9;"
   >
      blue 
   </strong>

components/back-top/demo/custom.md

@@ -23,7 +23,7 @@ ReactDOM.render(
       <div className="ant-back-top-inner">UP</div>
     </BackTop>
     Scroll down to see the bottom-right
-    <strong style={{ color: '#57c5f7' }}> blue </strong>
+    <strong style={{ color: '#1088e9' }}> blue </strong>
     button.
   </div>,
   mountNode
@@ -39,7 +39,7 @@ ReactDOM.render(
   width: 40px;
   line-height: 40px;
   border-radius: 4px;
-  background-color: #57c5f7;
+  background-color: #1088e9;
   color: #fff;
   text-align: center;
   font-size: 20px;