Fix backup/restore page to have proper headings

docs/admin/backup.md

-# Backup your instance
+# Backup/Restore your instance
+
+## Backup
 
 1. Stop the Pleroma service.
 2. Go to the working directory of Pleroma (default is `/opt/pleroma`)
@@ -6,7 +8,7 @@
 4. Copy `pleroma.pgdump`, `config/prod.secret.exs` and the `uploads` folder to your backup destination. If you have other modifications, copy those changes too.
 5. Restart the Pleroma service.
 
-## Restore your instance
+## Restore
 
 1. Stop the Pleroma service.
 2. Go to the working directory of Pleroma (default is `/opt/pleroma`)