docs: use latest core-js unpkg link (#8190)

Author: konsalex

packages/plugin-legacy/README.md

@@ -122,9 +122,9 @@ The legacy plugin offers a way to use native `import()` in the modern build whil
 
 Polyfill specifier strings for `polyfills` and `modernPolyfills` can be either of the following:
 
-- Any [`core-js` 3 sub import paths](https://unpkg.com/browse/core-js@3.8.2/) - e.g. `es/map` will import `core-js/es/map`
+- Any [`core-js` 3 sub import paths](https://unpkg.com/browse/core-js@latest/) - e.g. `es/map` will import `core-js/es/map`
 
-- Any [individual `core-js` 3 modules](https://unpkg.com/browse/core-js@3.8.2/modules/) - e.g. `es.array.iterator` will import `core-js/modules/es.array.iterator.js`
+- Any [individual `core-js` 3 modules](https://unpkg.com/browse/core-js@latest/modules/) - e.g. `es.array.iterator` will import `core-js/modules/es.array.iterator.js`
 
 **Example**