Fix makeNode method to reflect its implementation higher in the text

It makes the source to compile on top of that

fluent/model.md

@@ -151,7 +151,7 @@ final class User: Model {
         name = try node.extract("name")
     }
 
-    func makeNode() throws -> Node {
+    func makeNode(context: Context) throws -> Node {
         return try Node(node: [
             "id": id,
             "name": name