[libc++] Update link to Itanium C++ ABI documentation

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@373372 91177308-0d34-0410-b5e6-96231b3b80d8

include/cxxabi.h

diff --git a/include/cxxabi.h b/include/cxxabi.h
index c1271fd..d21d3e1 100644
--- a/include/cxxabi.h
+++ b/include/cxxabi.h
@@ -11,7 +11,7 @@
 
 /*
  * This header provides the interface to the C++ ABI as defined at:
- *       http://www.codesourcery.com/cxx-abi/
+ *       https://itanium-cxx-abi.github.io/cxx-abi/
  */
 
 #include <stddef.h>