Valid types description for a simple type in data

Since data is just a Symbol, types[:data] should just be described
as "Symbol". Array notation is meant to be used for describing
key => value pair.

Co-authored-by: Łukasz Reszke <[email protected]>

ruby_event_store/spec/mappers/transformation/preserve_types_spec.rb

@@ -305,7 +305,7 @@ module Transformation
             event_id: uuid,
             metadata: {
               types: {
-                data: ['Symbol', 'Symbol'],
+                data: 'Symbol',
               }
             },
             data: :any_given_symbol,