doc: update experimental loader hooks example code

It fix 2 issues in provided Loader hooks examples:
1. Original ``new URL(`${process.cwd()}/`, 'file://');``
is not cross-platform, it gives wrong URL on windows
2. Based on `CHECK` in ModuleWrap::Resolve (node 12.9.1,
https://github.com/nodejs/node/blob/v12.9.1/src/module_wrap.cc#L1132)
the 2nd parameter should be a `string`, not an `URL` object

PR-URL: https://github.com/nodejs/node/pull/29373


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>