docs(readme): Fix typo

[ExE-Boss]

This corrects:

numeric__px(Object.freeze({number: 3, string, "3"}))
//                                          ^

to

numeric__px(Object.freeze({number: 3, string: "3"}))
//                                          ^

---

Turns out, this isn’t the first time this has happened: https://github.com/tc39/proposal-extended-numeric-literals/pull/10 🤦🏻‍♂️

[littledan]

Thanks!