Rewrite Epic tier move message

...to follow the style guide

doc/api/epics.md

 # Epics API **(PREMIUM)**
 
+> - Introduced in [GitLab Ultimate](https://about.gitlab.com/pricing/) 10.2.
+> - Single-level Epics [were moved](https://gitlab.com/gitlab-org/gitlab/issues/37081) to [GitLab Premium](https://about.gitlab.com/pricing/) in 12.8.
+
 Every API call to epic must be authenticated.
 
 If a user is not a member of a group and the group is private, a `GET` request on that group will result to a `404` status code.

doc/user/group/epics/index.md

@@ -5,7 +5,7 @@ type: reference, howto
 # Epics **(PREMIUM)**
 
 > - Introduced in [GitLab Ultimate](https://about.gitlab.com/pricing/) 10.2.
-> - In [GitLab 12.8](https://gitlab.com/gitlab-org/gitlab/issues/37081), single-level Epics were moved to the Premium tier.
+> - Single-level Epics [were moved](https://gitlab.com/gitlab-org/gitlab/issues/37081) to [GitLab Premium](https://about.gitlab.com/pricing/) in 12.8.
 
 Epics let you manage your portfolio of projects more efficiently and with less
 effort by tracking groups of issues that share a theme, across projects and