[lib] Correct creatorID in DMOp validators to tUserID

Summary:
I noticed that I accidentally used t.String here instead of tUserID. I had copied the entry validators, which also had the issue.

Depends on D13433

Test Plan: I reviewed all uses of t.String in this file to make sure there were no other cases

Reviewers: will, tomek

Reviewed By: tomek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13434