squash: fix compiler warning 07_passing_wrapped_ob

Currently the following compiler warning is emitted:
1 warning generated.
../addon.cc:24:16:
warning: 'ToObject' is deprecated:
Use maybe version [-Wdeprecated-declarations]
      args[0]->ToObject(isolate));
               ^
./deps/v8/include/v8.h:2539:3:
note: 'ToObject' has been explicitly marked deprecated here
  V8_DEPRECATED("Use maybe version",
  ^

This commit updates example to use the non-deprecated version.

doc/api/addons.md

@@ -1081,6 +1081,7 @@ that can take two `MyObject` objects as input arguments:
 
 namespace demo {
 
+using v8::Context;
 using v8::FunctionCallbackInfo;
 using v8::Isolate;
 using v8::Local;
@@ -1095,11 +1096,12 @@ void CreateObject(const FunctionCallbackInfo<Value>& args) {
 
 void Add(const FunctionCallbackInfo<Value>& args) {
   Isolate* isolate = args.GetIsolate();
+  Local<Context> context = isolate->GetCurrentContext();
 
   MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
-      args[0]->ToObject(isolate));
+      args[0]->ToObject(context).ToLocalChecked());
   MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
-      args[1]->ToObject(isolate));
+      args[1]->ToObject(context).ToLocalChecked());
 
   double sum = obj1->value() + obj2->value();
   args.GetReturnValue().Set(Number::New(isolate, sum));