style: indent subsequent theorem statement (#698)

Std/Data/List/Lemmas.lean

@@ -161,7 +161,7 @@ theorem cons_eq_append :
   rw [eq_comm, append_eq_cons]
 
 theorem append_eq_append_iff {a b c d : List α} :
-  a ++ b = c ++ d ↔ (∃ a', c = a ++ a' ∧ b = a' ++ d) ∨ ∃ c', a = c ++ c' ∧ d = c' ++ b := by
+    a ++ b = c ++ d ↔ (∃ a', c = a ++ a' ∧ b = a' ++ d) ∨ ∃ c', a = c ++ c' ∧ d = c' ++ b := by
   induction a generalizing c with
   | nil => simp; exact (or_iff_left_of_imp fun ⟨_, ⟨e, rfl⟩, h⟩ => e ▸ h.symm).symm
   | cons a as ih => cases c <;> simp [eq_comm, and_assoc, ih, and_or_left]