fix homepage link background color (#146)

Author: kermanx

_includes/banner.html

@@ -16,7 +16,7 @@
           <i class="fab fa-gitlab fa-5x" aria-hidden="true"></i>
           <span>GitLab</span>
         </a>
-        <a href="https://201.ustclug.org/" class="bg-microsoft-green">
+        <a href="https://201.ustclug.org/" class="bg-microsoft-yellow">
           <i class="fad fa-graduation-cap fa-5x" aria-hidden="true"></i>
           <span>Linux 201</span>
         </a>