fix(readme): wrong file name

[GeopJr]

The purpose of this PR is:

There's a tiny typo on the module path in the readme examples . This PR changes the path so it points to the correct file!

This is what had to change:

- -> . (formdata-polyfill/esm-min.js -> formdata-polyfill/esm.min.js)

---

• [X] I prefixed the PR-title with docs:, fix(area):, feat(area): or breaking(area):
• [X] I updated the README.md

[jimmywarting]

thx