Dead link for usage example

[stes]

This link

https://github.com/serlo/frontend/blob/8fc5a2f9ffd418f427a4be70763797defbec4c42/packages/editor/README.md?plain=1#L19C60-L19C150

is currently dead; and the repo it links to seems deprecated (https://github.com/serlo/serlo-editor-for-edusharing/tree/main).