message: Replace outdated url of Cookiecuter.dockerfile
  Clicked the URL & it was outdated due to this change:
  No update or verification of the actual documenation content
diff: @@ -12,7 +12,7 @@ Using Cookiecutter extensions is a two-step process:
  This step depends on how the scaffolder is setup to use Cookiecutter:
  - Using a local Cookiecutter, or
  -- Using a Cookiecutter Docker image, e.g. [spotify/backstage-cookiecutter](https://github.com/backstage/backstage/blob/37e35b91/plugins/scaffolder-backend/scripts/Cookiecutter.dockerfile).
  +- Using a Cookiecutter Docker image, e.g. [spotify/backstage-cookiecutter](https://github.com/backstage/backstage/blob/master/plugins/scaffolder-backend/scripts/Cookiecutter.dockerfile).
  Say we want to install [`jinja2_custom_filters_extension`](https://pypi.org/project/jinja2-custom-filters-extension/) to use the `upper_case_first_letter` filter in a Cookiecutter template.
  