message: Fix create-an-app.md output image
  The image src url is broken.
diff: @@ -22,7 +22,7 @@ This will create a new Backstage App inside the current folder. The name of the
  app-folder is the name that was provided when prompted.
  <p align='center'>
  - <img src='https://github.com/spotify/backstage/raw/master/docs/create-app_output.png' width='600' alt='create app'>
  + <img src='https://github.com/spotify/backstage/raw/master/docs/getting-started/create-app_output.png' width='600' alt='create app'>
  </p>
  Inside that directory, it will generate all the files and folder structure
  