Remove methods from intro example code

I'd like to keep this first code snippet as focused as possible on the
most important concepts. We'll need to figure out somewhere else to put
exhaustive documentation of the full feature set.
diff --git a/demo-cxx/demo.h b/demo-cxx/demo.h
index 885293f..fafc474 100644
--- a/demo-cxx/demo.h
+++ b/demo-cxx/demo.h
@@ -12,13 +12,12 @@
   ~ThingC();
 
   std::string appname;
-
-  const std::string &get_name() const;
 };
 
 struct SharedThing;
 
 std::unique_ptr<ThingC> make_demo(rust::Str appname);
+const std::string &get_name(const ThingC &thing);
 void do_thing(SharedThing state);
 
 } // namespace example