doc: the Napi::ObjectWrap example does not compile

The current example does not compile using GCC 4.8.

PR-URL: nodejs/node-addon-api#339
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@NickNaso.local>

napi.h

@@ -1407,8 +1407,8 @@ namespace Napi {
   ///       public:
   ///         static void Initialize(Napi::Env& env, Napi::Object& target) {
   ///           Napi::Function constructor = DefineClass(env, "Example", {
-  ///             InstanceAccessor("value", &GetSomething, &SetSomething),
-  ///             InstanceMethod("doSomething", &DoSomething),
+  ///             InstanceAccessor("value", &Example::GetSomething, &Example::SetSomething),
+  ///             InstanceMethod("doSomething", &Example::DoSomething),
   ///           });
   ///           target.Set("Example", constructor);
   ///         }