update guide 5.4-6.0 references wrong branch

Created by: hikari-boulders

the init-script on github is taken from the stable-6.0 branch, but the init script on

https://gitlab.com/gitlab-org/gitlab-ce/blob/6-0-stable/doc/update/5.4-to-6.0.md

references the master branch (in section 7: Update Init Script)