message: docs: update yarnrc reference in project structure
diff: @@ -212,7 +212,7 @@ future.
  common for companies to have their own npm registry, and this file makes sure
  that this folder always uses the public registry.
  -- [`.yarnrc`](https://github.com/backstage/backstage/tree/master/.yarnrc) -
  +- [`.yarnrc.yml`](https://github.com/backstage/backstage/tree/master/.yarnrc.yml) -
  Enforces "our" version of Yarn.
  - [`app-config.yaml`](https://github.com/backstage/backstage/tree/master/app-config.yaml) -
  