replace tns-core-modules with @nativescript/core

[madmas]

PR Checklist

• [x ] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.

What is the current state of the documentation article?

use the scoped import of @nativescript/core instead of the old one

What is the new state of the documentation article?

improve sample by shoing the new import statements