[class.virtual] Fix comment in example

Author: jensmaurer

source/derived.tex

@@ -944,7 +944,7 @@
 }
 
 struct C : A, B {
-  virtual void g();             // ill-formed, no diagnostic required
+  virtual void g();             // error: preconditions of overridden functions are not the same
 };
 \end{codeblock}
 \end{example}