message: fixed backstage clone url instruction in the contributors page
diff: @@ -15,7 +15,7 @@ your own GitHub account and clone that code to your local machine or you can
  grab the one for the origin like so:
  ```bash
  -git clone git@github.com/backstage/backstage --depth 1
  +git clone git@github.com:backstage/backstage --depth 1
  ```
  If you cloned a fork, you can add the upstream dependency like so:
  