6.2 branch name in 6.2 to 6.3 update guide

Created by: ccpz

Hello,

This line in https://github.com/gitlabhq/gitlabhq/blob/master/doc/update/6.2-to-6.3.md

git diff 6.2-stable:config/gitlab.yml.example 6-3-stable:config/gitlab.yml.example

needs change 6.2-stable to 6-2-stable