test: fix compiler warning in doc/api/addons.md

Currently there are compiler warnings is emitted:
../addon.cc:17:58:
warning: 'ToString' is deprecated:
Use maybe version [-Wdeprecated-declarations]
obj->Set(String::NewFromUtf8(isolate, "msg"),
                             args[0]->ToString(isolate));
                                                         ^
deps/v8/include/v8.h:2537:3:
note: 'ToString' has been explicitly marked deprecated here
  V8_DEPRECATED("Use maybe version",
  ^

This commit updates examples to use the non-deprecated versions.

PR-URL: #23323
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/addons.md

@@ -587,6 +587,7 @@ property `msg` that echoes the string passed to `createObject()`:
 
 namespace demo {
 
+using v8::Context;
 using v8::FunctionCallbackInfo;
 using v8::Isolate;
 using v8::Local;
@@ -596,9 +597,11 @@ using v8::Value;
 
 void CreateObject(const FunctionCallbackInfo<Value>& args) {
   Isolate* isolate = args.GetIsolate();
+  Local<Context> context = isolate->GetCurrentContext();
 
   Local<Object> obj = Object::New(isolate);
-  obj->Set(String::NewFromUtf8(isolate, "msg"), args[0]->ToString(isolate));
+  obj->Set(String::NewFromUtf8(isolate, "msg"),
+                               args[0]->ToString(context).ToLocalChecked());
 
   args.GetReturnValue().Set(obj);
 }
@@ -1078,6 +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;
@@ -1092,11 +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));