Fix branch name for update docs

doc/update/5.4-to-6.0.md

@@ -36,7 +36,7 @@ sudo -u git -H RAILS_ENV=production bundle exec rake gitlab:backup:create
 ```bash
 cd /home/git/gitlab
 sudo -u git -H git fetch
-sudo -u git -H git checkout 6-0-dev
+sudo -u git -H git checkout 6-0-stable
 ```
 
 ### 3. Install additional packages