Fix a mistake in specs for transactional #update_fields

Author: andrykonchin

spec/dynamoid/transaction_write/update_fields_spec.rb

@@ -88,7 +88,7 @@
 
         expect {
           described_class.execute do |t|
-            t.update_fields klass_with_composite_key, nil, name: 'Alex [Updated]'
+            t.update_fields klass, nil, name: 'Alex [Updated]'
           end
         }.to raise_exception(Dynamoid::Errors::MissingHashKey)
       end