Fix broken link pointing to the 12.8 release page

Why is this change being made?

Fixing a broken link to the 12.8 releases page.

broken url: https://about.gitlab.com/2020/02/22/gitlab-12-8-released/
correct url: https://about.gitlab.com/releases/2020/02/22/gitlab-12-8-released/

Does this MR meet the acceptance criteria?

Yes

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Assigned to author of the release post.

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what