chore: add missing instance (#8772)

Changes `ReverseImplicationOrder.instCompleteLattice` to be an
`instance`.

Author: cppio

src/Init/Internal/Order/Basic.lean

@@ -914,7 +914,7 @@ instance ReverseImplicationOrder.instOrder : PartialOrder ReverseImplicationOrde
   rel_antisymm h₁ h₂ := propext ⟨h₂, h₁⟩
 
 -- This defines a complete lattice on `Prop`, used to define coinductive predicates
-def ReverseImplicationOrder.instCompleteLattice : CompleteLattice ReverseImplicationOrder where
+instance ReverseImplicationOrder.instCompleteLattice : CompleteLattice ReverseImplicationOrder where
   sup c := ∀ p, c p → p
   sup_spec := by
     intro x c