fix(Topology/Baire/Lemmas): fix index type in isMeagre_iUnion (#30141)

The index set ι needs to be of type Sort* instead of type Type* in isMeagre_iUnion otherwise I get the following message when I try to use it:
typeclass instance problem is stuck, it is often due to metavariables
  Countable ?m.27

Author: shalliso

Mathlib/Topology/GDelta/Basic.lean

@@ -232,7 +232,7 @@ lemma IsMeagre.union {s t : Set X} (hs : IsMeagre s) (ht : IsMeagre t) : IsMeagr
   exact inter_mem hs ht
 
 /-- A countable union of meagre sets is meagre. -/
-lemma isMeagre_iUnion [Countable ι] {f : ι → Set X} (hs : ∀ i, IsMeagre (f i)) :
+lemma isMeagre_iUnion [Countable ι'] {f : ι' → Set X} (hs : ∀ i, IsMeagre (f i)) :
     IsMeagre (⋃ i, f i) := by
   rw [IsMeagre, compl_iUnion]
   exact countable_iInter_mem.mpr hs