Fix webpack config example in readme

[njam]

I'm really a webpack noob, so please forgive me if I'm missing something obvious here. But according to the docs the key to define rules is module.rules?

This PR contains a:

• [ ] bugfix
• [ ] new feature
• [ ] code refactor
• [ ] test update
• [ ] typo fix
• [x] documentations
• [ ] metadata update

Motivation / Use-Case

npx webpack wouldn't run with the given example config

Breaking Changes

none

Additional Info