fix required gitlab-shell version.

Under the procedure, I've warned about gitlab-shell version, which should be 2.6.12 (may be some minor revisions introduces that).

doc/update/8.5-to-8.6.md

@@ -46,7 +46,7 @@ sudo -u git -H git checkout 8-6-stable-ee
 ```bash
 cd /home/git/gitlab-shell
 sudo -u git -H git fetch --all
-sudo -u git -H git checkout v2.6.11
+sudo -u git -H git checkout v2.6.12
 ```
 
 ### 5. Update gitlab-workhorse