Add trailing comma in multiline RBS translation

[jesse-shopify]

      sig do
        params(backend_name: Symbol, api_surface: Symbol)
          .returns([T.any(LegacySearch, ShopLensSearch, FullContextSearch), T::Array[T::Hash[Symbol, T.untyped]]])
      end

Is currently:

      #: (
      #|   Symbol backend_name,
      #|   Symbol api_surface
      #| ) -> [(LegacySearch | ShopLensSearch | FullContextSearch), Array[Hash[Symbol, untyped]]]

but could be:

      #: (
      #|   Symbol backend_name,
      #|   Symbol api_surface,
      #| ) -> [(LegacySearch | ShopLensSearch | FullContextSearch), Array[Hash[Symbol, untyped]]]

Requested by @amomchilov