Update installation.md

Currently, when following these instructions, you get a warning during final check.

Git configured for git user? ... no
  Try fixing it:
  sudo -u git -H git config --global user.name  "GitLab"
  sudo -u git -H git config --global user.email "gitlab@your-domain.com"
  For more information see:
  doc/install/installation.md in section "GitLab"
  Please fix the error above and rerun the checks.

To fix this, have user.name and user.email set for the new git user.