erp5_core: Fix Workflow Initial State relation string field.

Use correct portal type.
7 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:41:20

passed ERP5.PerformanceTest-Master

00:26:48

failed ERP5.UnitTest-Master

01:28:48

failed ERP5.UnitTest-Master.Medusa

08:39:06

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:19

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:15

failed ERP5.UnitTest-Master.Medusa

01:35:39