Remove duplicate line in Administration guide

Running-Mastodon/Administration-guide.md

@@ -8,7 +8,6 @@ So, you have a working Mastodon instance... now what?
 The following rake task:
 
     RAILS_ENV=production bundle exec rails mastodon:make_admin USERNAME=alice
-    (or docker-compose run --rm web rails mastodon:make_admin USERNAME=alice)
 
 or, if using docker: