doc: fix return value for require.resolve.paths()

Update documentation of modules.md for require.resolve.paths()
to include the possibillity of null as a return value if
the request string given matches a core module name.

PR-URL: https://github.com/nodejs/node/pull/18350
Fixes: https://github.com/nodejs/node/issues/18230


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>