doc: require() tries first core not native modules

Change a single word in documentation with a more precise one.

Native is a module compiled in machine "native" code.
A module normally written in a compiled language, not in JavaScript.

Core modules form Node's built-in "core" functionalities.
You don't need to install them. They are included in every Node installation
and documented in https://nodejs.org/api/ .

PR-URL: https://github.com/nodejs/node/pull/10324


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>