[#275] netmap: Fix storage model docs

Actualize storage node's structure type and clarify this is a type not
to cause misunderstanding.

Signed-off-by: Leonard Lyubich <ctulhurider@gmail.com>

netmap/doc.go

@@ -44,12 +44,12 @@ Key-value storage format:
    block which "ticked" the current epoch
  - 'snapshotCount' -> int
    number of stored network maps including current one
- - 'snapshot_<ID>' -> std.Serialize([]storageNode)
-   network map by snapshot ID
+ - 'snapshot_<ID>' -> std.Serialize([]Node)
+   network map by snapshot ID (where Node is a type)
  - 'snapshotCurrent' -> int
    ID of the snapshot representing current network map
- - 'candidate<public_key>' -> std.Serialize(netmapNode)
-   information about the particular network map candidate
+ - 'candidate<public_key>' -> std.Serialize(Node)
+   information about the particular network map candidate (where Node is a type)
  - 'containerScriptHash' -> 20-byte script hash
    Container contract reference
  - 'balanceScriptHash' -> 20-byte script hash