bump versions in update guide

doc/update/6.7-to-6.8.md

@@ -21,7 +21,7 @@ sudo -u git -H git fetch --all
 For Gitlab Community Edition:
 ```bash
-sudo -u git -H git checkout 6-7-stable
+sudo -u git -H git checkout 6-8-stable
 ```
 OR
@@ -29,7 +29,7 @@ OR
 For GitLab Enterprise Edition:
 ```bash
-sudo -u git -H git checkout 6-7-stable-ee
+sudo -u git -H git checkout 6-8-stable-ee
 ```
 ### 3. Update gitlab-shell (and its config)