Fix instance variable name typo

Author: kamipo

activerecord/lib/active_record/transactions.rb

@@ -452,7 +452,7 @@ def trigger_transactional_callbacks? # :nodoc:
       def init_internals
         super
         @_start_transaction_state = nil
-        @_last_transaction_status = nil
+        @_last_transaction_return_status = nil
         @_committed_already_called = nil
         @_new_record_before_last_commit = nil
       end