doc: fix deprecation warnings in addon examples

Use the overload of `v8::Function::NewInstance()` that returns a
`v8::MaybeLocal<v8::Object>`.  The overloads that return a simple
`v8::Local<v8::Object>` are deprecated.

PR-URL: https://github.com/nodejs/node/pull/6652


Reviewed-By: Anna Henningsen <anna@addaleax.net>