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change type for mod attribut to allow for arbitrary arguments

Summary:
We currently assume that ars is a list, but that's not correct. We can pass anything as args.

This change sets the type to string to allow arbitrary terms as second argument for the mod attribute.

Reviewed By: michalmuskala

Differential Revision: D70266532

fbshipit-source-id: 1cd75ccbeb86f38f27cbf0e1c409221414e91e08