Update 6.9-to-7.0.md

fix gitlab.yml example diff command

doc/update/6.9-to-7.0.md

@@ -103,7 +103,7 @@ sudo chmod +x /etc/init.d/gitlab
 There are new configuration options available for gitlab.yml. View them with the command below and apply them to your current gitlab.yml if desired.
 
 ```
-git diff 6-9-stable:config/gitlab.yml.example 7-0-stable:config/gitlab.yml.example
+git diff origin/6-9-stable:config/gitlab.yml.example origin/7-0-stable:config/gitlab.yml.example
 ```
 
 ### 7. Start application