message: docs: update frontend assets path in debugging doc
  Frontend code has been moved by
diff: @@ -13,8 +13,7 @@ npm link
  2. Link the local package to `incubator-superset`.
  ```sh
  -cd incubator-superset
  -cd superset/assets
  +cd incubator-superset/superset-frontend
  npm link @superset-ui/chart \# use package name in package.json, not directory name
  ```
  @@ -49,8 +48,8 @@ inside `incubator-superset`.
  2. Unlink the package from `incubator-superset`.
  -```cd incubator-superset
  -cd superset/assets
  +```sh
  +cd incubator-superset/superset-frontend
  npm unlink @superset-ui/chart
  ```
  