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

Checklist

• documentation is changed or added
• commit message follows commit guidelines