Tiny readme fix for import-map example code.

[Bug-Reaper]

Corrects example so the import-map path is relative.

Copy and paste current import-map example code it's gonna trigger an error:

Also changes tween.es.js => tween.esm.js. There does not seem to be any tween.es.js available in the default npm install so assume this is a typo.

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The importmap example code works for me after these two small fixes ✌️

[trusktr]

Thank you!