Remove the link to polyfill.io, which is found to serve malware

[KonerDev]

Describe the bug

On the page https://javascript.info/polyfills there is a link to polyfill.io, which is found to serve malware because the site was sold to a Chinese company. Instead, replace it with a link from the Github repository, which as far as I know is not affected: https://github.com/polyfillpolyfill/polyfill-service

The polyfill.io site is currently offline.

Sources

https://www.bleepingcomputer.com/news/security/polyfillio-javascript-supply-chain-attack-impacts-over-100k-sites/
https://sansec.io/research/polyfill-supply-chain-attack

[CSKishan]

I've raised a PR for this