Update: footer logo (#1550)

Author: AmosHuKe

src/_includes/footer.html

@@ -1,7 +1,7 @@
 <footer id="site-footer">
   <div class="footer-section footer-main">
     <a class="brand" href="https://flutter.cn">
-      <img src="/assets/images/branding/flutter/logo+text/horizontal/white.svg" alt="Flutter logo" width="164">
+      <img src="/assets/images/cn/flutter-cn-logo-text-horizontal-white.svg" alt="Flutter logo" width="164">
     </a>
     <div class="footer-social-links">
       <a class="icon-button" href="https://medium.com/flutter" target="_blank" rel="noopener" title="Flutter's Medium blog">