Update alt text of adative concurrency limit graph

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/170900



Merged-by: Evan Read <eread@gitlab.com>
Approved-by: Justin Tobler <jtobler@gitlab.com>
Approved-by: Evan Read <eread@gitlab.com>
Reviewed-by: Justin Tobler <jtobler@gitlab.com>
Co-authored-by: Mansoor Khan <thisismansoorkhan@gmail.com>
Co-authored-by: Mansoor Khan <mansoor@incubyte.co>