fix test

Author: kim-em

tests/lean/run/grind_getElem.lean

@@ -6,5 +6,5 @@ attribute [grind]
   List.getElem_cons
   List.getElem?_cons List.getElem?_nil
 
-theorem getElem?_eq_some_iff {l : List α} : l[i]? = some a ↔ ∃ h : i < l.length, l[i] = a := by
+example {l : List α} : l[i]? = some a ↔ ∃ h : i < l.length, l[i] = a := by
   induction l generalizing i <;> grind