Text and tone improvements for custom workspace image tutorial

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/184048



Merged-by: Isaac Durham <idurham@gitlab.com>
Approved-by: Isaac Durham <idurham@gitlab.com>
Reviewed-by: Isaac Durham <idurham@gitlab.com>
Co-authored-by: Brendan <blynch@gitlab.com>