Need to update README.md

[jheelbharambe]

Brief bug description

Please update the README clean in code part here :https://github.com/Kentico/kontent-backup-manager-js#clean-in-code
This references to ImportService instead of CleanService.

What went wrong?

Repro steps

1. Go to https://github.com/Kentico/kontent-backup-manager-js/blob/master/README.md
2. Scroll down to https://github.com/Kentico/kontent-backup-manager-js/blob/master/README.md
3. See const importService = new ImportService({... This should be CleanService

Expected behavior

What the correct behavior is?

Test environment

• Platform/OS: [e.g. .NET Core 2.1, iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Additional context

Add any other context about the problem here.

Screenshots

Add links to screenshots, if possible.