Update broken migration link 

There is a broken link that is accessible through [this page](https://www.patternfly.org/extensions/component-groups/about-component-groups) (you can refer to this migration guide text).

It is currently trying to navigate to https://github.com/patternfly/react-component-groups/migration.md and should be navigating to https://github.com/patternfly/react-component-groups/blob/main/migration.md