manual: Fix RST reference to MS extension support

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190101 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/UsersManual.rst b/docs/UsersManual.rst
index d2ded6b..2dc6068 100644
--- a/docs/UsersManual.rst
+++ b/docs/UsersManual.rst
@@ -1341,7 +1341,7 @@
 
 Experimental supports are on Cygming.
 
-See also `Microsoft Extensions <c_ms>`.
+See also :ref:`Microsoft Extensions <c_ms>`.
 
 Cygwin
 """"""