fix nesting on snapshot repo topics

[shainaraskas]

other snapshot repo topics have bad nesting of headings - they're nested under a 7.x prereq which is not needed

consider deleting the 7.x sections completely in this set of docs

example fix here: #2435

another example broken topic: https://www.elastic.co/docs/deploy-manage/tools/snapshot-and-restore/ec-azure-snapshotting