Fix group name in memberships export email

What does this MR do and why?

Resolves an issue where the group name wouldn't render in the group membership export email body as described in #367612 (closed).

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to the Group > Members page at http://localhost:3000/groups/gitlab-org/-/group_members.
2. Click on Export as CSV.
3. View the email in rails letter opener at http://127.0.0.1:3000/rails/letter_opener/.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #367612 (closed)