Update diagnostic on compatibility page to reflect reality

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107382 91177308-0d34-0410-b5e6-96231b3b80d8

www/compatibility.html

@@ -590,9 +590,8 @@ void g(Base *base) {
 downcast.mm:6:3: error: no matching function for call to 'f'
   f(base);
   ^
-downcast.mm:4:6: note: candidate function not viable: cannot convert from base
-      class pointer 'Base *' to derived class pointer 'Derived *' for 1st
-      argument
+downcast.mm:4:6: note: candidate function not viable: cannot convert from
+      superclass 'Base *' to subclass 'Derived *' for 1st argument
 void f(Derived *);
      ^
 </pre>