Add dot to GraphQL descriptions of epic_type.rb

What does this MR do?

This MR adds dot to GraphQL descriptions of ee/app/graphql/types/epic_type.rb. A follow-up that was suggested in !52922 (merged)

Mentions #300798 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content