Update segments spec

lib/plausible/segment/segments.md

@@ -59,7 +59,7 @@ __*__: if the user has that particular capability
 |-------|------|-------------|---------|
 | :id | :bigint | null: false | |
 | :name | :string | null: false | |
-| :personal | :boolean | default: true, null: false | Needed to distinguish between segments that are supposed to be listed site-wide and ones that are listed only for author |
+| :type | :enum | default: :personal, null: false | Possible values are :site, :personal. Needed to distinguish between segments that are supposed to be listed site-wide and ones that are listed only for author |
 | :segment_data | :map | null: false | Contains the filters array at "filters" key and the labels record at "labels" key |
 | :site_id | references(:sites) | on_delete: :delete_all, null: false | |
 | :owner_id | references(:users) | on_delete: :nothing, null: false | Used to display author info without repeating author name and email in the database |