[Branch-Rename] Fix some links

According to the [[ https://foundation.llvm.org/docs/branch-rename/ | status of branch rename ]], the master branch of the LLVM repository is removed on 28 Jan 2021.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D95766

GitOrigin-RevId: 94fac81fccfef9917e94bed398781744fb82e159
diff --git a/www/index.html b/www/index.html
index b8fd20f..b6275e6 100644
--- a/www/index.html
+++ b/www/index.html
@@ -26,7 +26,7 @@
     <a href="https://lists.llvm.org/mailman/listinfo/libcxx-dev">libcxx-dev</a>
     <a href="https://lists.llvm.org/mailman/listinfo/libcxx-commits">libcxx-commits</a>
     <a href="https://bugs.llvm.org/">Bug Reports</a>
-    <a href="https://github.com/llvm/llvm-project/tree/master/libcxxabi/">Browse Sources</a>
+    <a href="https://github.com/llvm/llvm-project/tree/main/libcxxabi/">Browse Sources</a>
   </div>
 </div>