Docs fix for omniauth sync_profile_ attributes

What does this MR do?

Closes gitlab-com/support/docs#58

As seen in the issue above ☝, the current docs refer to omniauth sync_profile_ attributes in Omnibus, whereas the Omnibus code checks for the attributes prefixed with omniauth_.

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)