docs: fix dead link (#15700)

Author: ryuji-1to

packages/plugin-legacy/README.md

@@ -87,7 +87,7 @@ npm add -D terser
 
   Note it is **not recommended** to use the `true` value (which uses auto-detection) because `core-js@3` is very aggressive in polyfill inclusions due to all the bleeding edge features it supports. Even when targeting native ESM support, it injects 15kb of polyfills!
 
-  If you don't have hard reliance on bleeding edge runtime features, it is not that hard to avoid having to use polyfills in the modern build altogether. Alternatively, consider using an on-demand service like [Polyfill.io](https://polyfill.io/v3/) to only inject necessary polyfills based on actual browser user-agents (most modern browsers will need nothing!).
+  If you don't have hard reliance on bleeding edge runtime features, it is not that hard to avoid having to use polyfills in the modern build altogether. Alternatively, consider using an on-demand service like [Polyfill.io](https://polyfill.io) to only inject necessary polyfills based on actual browser user-agents (most modern browsers will need nothing!).
 
 ### `renderLegacyChunks`