Update polyfill link

[iosh]

The usage of polyfill.io is not secure. Please refer to the following link for details : https://sansec.io/research/polyfill-supply-chain-attack

So switch to Fastly. More information can be found here : https://community.fastly.com/t/new-options-for-polyfill-io-users/2540

[google-cla]

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[niooss-ledger]

Hello,
[I'm just someone who searched polyfill.io on GitHub Search and found this Pull Request, I'm not related to this project]
Your Pull Request only modifies README.md, but its content seems to be generated from site/src/api.njk:

quicklink/site/src/api.njk

Lines 193 to 197 in cd9ba8e

	- Requires `IntersectionObserver` to be supported (see [Can I Use](https://caniuse.com/intersectionobserver)). We recommend conditionally polyfilling this feature with a service like Polyfill.io:
	{% endmarkdownConvert %}
	{% highlight "html" %}
	<script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"></script>

I believe this file should also be updated.