DOC FIX bad link for copy-webpack-plugin (#7291)

The link to the copy-webpack-plugin release notes was wrong (copy-pasted from the "webpack-html-plugin" juste above)

docs/src/pages/quasar-cli/app-upgrade-guide.md

@@ -80,7 +80,7 @@ Following this guide should take you at most 5 minutes to complete.
 
 * The `webpack-html-plugin` package has been upgraded to v4; if you have tampered with the default config for it, please [review it](https://github.com/jantimon/html-webpack-plugin/blob/master/CHANGELOG.md#400-2020-03-23)
 
-* The `copy-webpack-plugin` package has been upgraded to v6; if you have tampered with the default config for it, please [review it](https://github.com/jantimon/html-webpack-plugin/blob/master/CHANGELOG.md#400-2020-03-23)
+* The `copy-webpack-plugin` package has been upgraded to v6; if you have tampered with the default config for it, please [review it](https://github.com/webpack-contrib/copy-webpack-plugin/blob/master/CHANGELOG.md#600-2020-05-15)
 
 * If you're building with SSR mode, then custom publicPath can now be used, but it requires a small change to all your app.use() statements:
   ``` js