Fix help message of user_unique_last_password_count

If we want to use identity.user_unique_last_password_count, we must
firstly set identity-feature-enabled.security_compliance to True,
whose default value is False.
So this is to fix the help message of user_unique_last_password_count
to mention this.

Change-Id: I0979c468b67957309a5c877f084eaf4c47007e6d
1 file changed