Clarify 'GitLab' term in signup error message

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/166904



Merged-by: Eduardo Sanz García <esanz-garcia@gitlab.com>
Approved-by: Javiera Tapia <jtapia@gitlab.com>
Approved-by: Aboobacker MK <akarakath@gitlab.com>
Reviewed-by: Eduardo Sanz García <esanz-garcia@gitlab.com>