Update instructions for installing GDK from own fork

Overview

Under the Install your own fork section in the alternative GDK installation methods documentation, the gdk install code is given as gdk install gitlab_repo=git@gitlab.com:gitlab-org/gitlab-foss.git where the gitlab_repo option references the official gitlab organisation name. Shouldn't this instead refer to the user's own fork? Such as below:

gdk install gitlab_repo=git@gitlab.com:<username>/gitlab-foss.git

Here username refers to the user's gitlab username where their fork would be and not the Gitlab Org's username.