Fix api-ref for GET snapshot response

GET snapshot without detail does not return user_id
atributes but api-ref say it does. Fixing the api-ref.

Change-Id: I006df750a9f01098c8c326648f5da472d1a10f06
Closes-Bug: #1878699

api-ref/source/v3/volumes-v3-snapshots.inc

@@ -199,7 +199,6 @@ Response Parameters
    - description: description_snap_req
    - created_at: created_at
    - name: name
-   - user_id: user_id_min
    - volume_id: volume_id_snap
    - metadata: metadata
    - id: id_snap