Update string structure for group runners

What does this MR do?

During reviewing https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24095#note_129340781, I noticed that some externalized strings using : were using some extra blank and full stop characters. Following the update in !23347 (merged), this will make similar updates in the string structure for available group runners in the CI/CD settings page.

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer