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: https://github.com/nodejs/node/pull/23323


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