Fix danger instructions for dev

On dev we don't have the
pipeline-common project,
so including it makes sense only on .com

doc/development/dangerbot.md

@@ -154,6 +154,8 @@ To enable the Dangerfile on another existing GitLab project, complete the follow
       - project: 'gitlab-org/quality/pipeline-common'
         file:
           - '/ci/danger-review.yml'
+        rules:
+          - if: '$CI_SERVER_HOST == "gitlab.com"'
     ```
 
 1. If your project is in the `gitlab-org` group, you don't need to set up any token as the `DANGER_GITLAB_API_TOKEN`