Update formatting re: marketing emails

What does this MR do?

In https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/308 (this comment) I was asked to take a look at the docs changes. What I saw: @alexbuijs had absolutely done the right thing, and copied formatting from another subheading on the same page … but that didn't make the formatting right.

This merge request contains two commits:

• A commit that directly addresses the changes Alex contributed to https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/308
• A "while I'm here, I might as well" quick tidying up of tone and style.

Related to https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/308

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