fix c.127 example issue. (#789)

auto will deduce as unique_ptr<D> and the correct destructor will call. Anyway, this should be a bad example.

CppCoreGuidelines.md

@@ -6084,7 +6084,7 @@ A class with a virtual function is usually (and in general) used via a pointer t
 
     void use()
     {
-        auto p = make_unique<D>();
+        unique_ptr<B> p = make_unique<D>();
         // ...
     } // undefined behavior. May call B::~B only and leak the string